TcpConnection uses a send_lock that's acquired before calling doSend().

This is not needed when we have a single sender thread, e.g. as in TransferQueueBundler, PerDestinationBundler etc.

Therefore allow for optional locking, e.g. to be enabled/disabled by a (single-thread) bundler at initialization time.

Additionally, field writers would not have to be incremented/decremented in TcpConnection.