Support etcd.v3 and new generic topology

[ochaton]

We need to stay up to date to tarantool main line. At least for migration from current topologies to proposed one

Moreover, due to lack of transactions in etcd/v2 we can't safely survive heavy disaster such as complete destruction of etcd cluster (rm -rf / or accidental shutdown)

[vanyarock01]

https://etcd.io/docs/v3.5/dev-guide/api_grpc_gateway/ can be used for that.