Remove the dependency of go.etcd.io/etcd, help bump grpc

[zhongzc]

Currently, every other go app which depends on tidb-dashboard should replace grpc version to less than v1.29.1, and it often causes dependency problems.

replace google.golang.org/grpc => google.golang.org/grpc v1.29.1

A good solution I found at pingcap/tidb#32457 may help. It replaced the dependency of go.etcd.io/etcd v0.0.0-20191023171146-3cf2f69b5738 to

go.etcd.io/etcd/api/v3 v3.5.2
go.etcd.io/etcd/client/pkg/v3 v3.5.2
go.etcd.io/etcd/client/v3 v3.5.2
go.etcd.io/etcd/server/v3 v3.5.2
go.etcd.io/etcd/tests/v3 v3.5.2