Prevent Socket.write() from blocking

[acogoluegnes]

Originally reported on rabbitmq-discuss

Java NIO could be a solution to blocking on writes, but it's a major rewriting of the client. This migration is considered on the mid-term, as it could bring features and reliability to the table.

In the meantime, a lighter solution to the problem mentioned above could be found, e.g. making part of the connection shutdown (where the blocking/deadlock occur) asynchronous.