Switch Addresses in proto files from bytes to string

[sunnya97]

In some of the modules' proto files, addresses are defined by bytes to be gogoproto casted to sdk.AccAddress

Example:

message MsgCreatePot {
  bool is_perpetual = 1; // flag to show if it's perpetual or multi-epoch
                         // distribution incentives by third party
  bytes owner = 2 [ (gogoproto.casttype) =
                        "github.com/cosmos/cosmos-sdk/types.AccAddress" ];

...

These should be switched to strings as this is the standard in the cosmos sdk now.