MEDIUM: buffer: remove the buffer_wq lock

MEDIUM: buffer: remove the buffer_wq lock

This lock was only needed to protect the buffer_wq list, but now we have
the mt_list for this. This patch simply turns the buffer_wq list to an
mt_list and gets rid of the lock.

It's worth noting that the whole buffer_wait thing still looks totally
wrong especially in a threaded context: the wakeup_cb() callback is
called synchronously from any thread and may end up calling some
connection code that was not expected to run on a given thread. The
whole thing should probably be reworked to use tasklets instead and be
a bit more centralized.