cbus: fix inconsistency in endpoint creation

The notification of wait variable shall be done under
a bound mutex locked. Otherwise the results are not
guaranteed (see pthread manuals).

Thus when we create a new endpoint via cbus_endpoint_create
and there is an other thread which sleeps inside cpipe_create
we should notify the sleeper under cbus.mutex.

Fixes #4806

Reported-by: Alexander Turenko <alexander.turenko@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>