etcdserver: refactor sender

1. restrict the number of inflight connections to remote member
2. support stop

The PR tries to minimize the code to reach the goals.

for #1205 
@jonboulle it could be optimized later. for example, streaming could make appendEntry always use one connection, and we could kill previous heartbeat if the following one comes.
For now, i think it is fine to throw it away and pretend that it is lost when transfer.