Work around for LIBZMQ-496

The problem is that other threads might still be in mailbox::send() when
it is destroyed. So as a workaround, we just acquire the mutex in the
destructor. Therefore the running send will finish before the mailbox is
destroyed.

See also the fix for LIBZMQ-281 in zeromq2-x.

Signed-off-by: Mika Fischer <mika.fischer@zoopnet.de>