Use a separate list and mutex for loggers.

Use a separate list and mutex for loggers.

This avoids deadlocks caused by extensive listener_t implementations
which might want to acquire a lock which is currently held by another
thread wanting to log messages. Since the latter requires that thread
to acquire the same lock the initial thread currently holds this
previously resulted in a deadlock.

With this change logging messages does not require threads to acquire
the main lock in bus_t and thus avoids the deadlock.