About the lock in ZMQVan::SendMsg

Might the granularity of the lock be smaller? I'm not sure.

I feel that reasons why the lock guards the whole function body are:
1) protect senders_ from racing
2) keep the data transfer to the socket continuously and avoid being interrupted

As for reason 1, smaller granularity is acceptable.
As for reason 2, perhaps other operations than locking could be tried.