Use single codepath for sending transaction for local and remote nodes

Previously procedure for sending transactions was different based on
which type of node was used, local (light node) or remote.

This change removes separate code path for working with light node,
and the only difference will be backend for rpc client. In case of
light node all of the communication will be done over inproc dial.

Also there is a couple of related changes in this PR:
- new EthereumTransactor that provides higher level API for working
  with ethereum network, and it is fully conformant with ethclient
- new test rpc service that improves flexibility and coverage of
  txqueue manager tests

geth/txqueue/ethtxclient.go

@@ -0,0 +1,93 @@
+package txqueue
+
+import (
+	"context"
+	"math/big"
+
+	ethereum "github.com/ethereum/go-ethereum"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/rlp"
+	"github.com/status-im/status-go/geth/rpc"
+)
+
+// EthereumTransactor provides methods to create transactions for ethereum network.
+type EthereumTransactor interface {
+	PendingNonceAt(ctx context.Context, account common.Address) (uint64, error)
+	ethereum.GasEstimator
+	ethereum.GasPricer
+	ethereum.TransactionSender
+}
+
+// EthTxClient wraps common API methods that are used to send transaction.
+type EthTxClient struct {
+	c *rpc.Client
+}
+
+func NewEthTxClient(client *rpc.Client) *EthTxClient {
+	return &EthTxClient{c: client}
+}
+
+// PendingNonceAt returns the account nonce of the given account in the pending state.
+// This is the nonce that should be used for the next transaction.
+func (ec *EthTxClient) PendingNonceAt(ctx context.Context, account common.Address) (uint64, error) {
+	var result hexutil.Uint64
+	err := ec.c.CallContext(ctx, &result, "eth_getTransactionCount", account, "pending")
+	return uint64(result), err
+}
+
+// SuggestGasPrice retrieves the currently suggested gas price to allow a timely
+// execution of a transaction.
+func (ec *EthTxClient) SuggestGasPrice(ctx context.Context) (*big.Int, error) {
+	var hex hexutil.Big
+	if err := ec.c.CallContext(ctx, &hex, "eth_gasPrice"); err != nil {
+		return nil, err
+	}
+	return (*big.Int)(&hex), nil
+}
+
+// EstimateGas tries to estimate the gas needed to execute a specific transaction based on
+// the current pending state of the backend blockchain. There is no guarantee that this is
+// the true gas limit requirement as other transactions may be added or removed by miners,
+// but it should provide a basis for setting a reasonable default.
+func (ec *EthTxClient) EstimateGas(ctx context.Context, msg ethereum.CallMsg) (*big.Int, error) {
+	var hex hexutil.Big
+	err := ec.c.CallContext(ctx, &hex, "eth_estimateGas", toCallArg(msg))
+	if err != nil {
+		return nil, err
+	}
+	return (*big.Int)(&hex), nil
+}
+
+// SendTransaction injects a signed transaction into the pending pool for execution.
+//
+// If the transaction was a contract creation use the TransactionReceipt method to get the
+// contract address after the transaction has been mined.
+func (ec *EthTxClient) SendTransaction(ctx context.Context, tx *types.Transaction) error {
+	data, err := rlp.EncodeToBytes(tx)
+	if err != nil {
+		return err
+	}
+	return ec.c.CallContext(ctx, nil, "eth_sendRawTransaction", common.ToHex(data))
+}
+
+func toCallArg(msg ethereum.CallMsg) interface{} {
+	arg := map[string]interface{}{
+		"from": msg.From,
+		"to":   msg.To,
+	}
+	if len(msg.Data) > 0 {
+		arg["data"] = hexutil.Bytes(msg.Data)
+	}
+	if msg.Value != nil {
+		arg["value"] = (*hexutil.Big)(msg.Value)
+	}
+	if msg.Gas != nil {
+		arg["gas"] = (*hexutil.Big)(msg.Gas)
+	}
+	if msg.GasPrice != nil {
+		arg["gasPrice"] = (*hexutil.Big)(msg.GasPrice)
+	}
+	return arg
+}

geth/txqueue/fake/txservice.go

@@ -0,0 +1,40 @@
+package fake
+
+import (
+	context "context"
+	big "math/big"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/rpc"
+	gomock "github.com/golang/mock/gomock"
+)
+
+func NewTestServer(ctrl *gomock.Controller) (*rpc.Server, *MockFakePublicTxApi) {
+	srv := rpc.NewServer()
+	svc := NewMockFakePublicTxApi(ctrl)
+	if err := srv.RegisterName("eth", svc); err != nil {
+		panic(err)
+	}
+	return srv, svc
+}
+
+// CallArgs copied from module go-ethereum/internal/ethapi
+type CallArgs struct {
+	From     common.Address  `json:"from"`
+	To       *common.Address `json:"to"`
+	Gas      hexutil.Big     `json:"gas"`
+	GasPrice hexutil.Big     `json:"gasPrice"`
+	Value    hexutil.Big     `json:"value"`
+	Data     hexutil.Bytes   `json:"data"`
+}
+
+// FakePublicTxApi used to generate mock by mockgen util.
+// This was done because PublicTransactionPoolAPI is located in internal/ethapi module
+// and there is no easy way to generate mocks from internal modules.
+type FakePublicTxApi interface {
+	GasPrice(ctx context.Context) (*big.Int, error)
+	EstimateGas(ctx context.Context, args CallArgs) (*hexutil.Big, error)
+	GetTransactionCount(ctx context.Context, address common.Address, blockNr rpc.BlockNumber) (*hexutil.Uint64, error)
+	SendRawTransaction(ctx context.Context, encodedTx hexutil.Bytes) (common.Hash, error)
+}