fix: lock the connection during a 'notifies()' call

fix: lock the connection during a 'notifies()' call

With the previous implementation, it was possible to sneak an execute()
while the generator is consumed. This gives the false impression that
it's possible to use the connection while listening (see #756), which is
false for reason better explored in #757.

Therefore, lock the connection while listening to notifications. If
someone wants to mix commands with listening on the same connection,
they should do it collaboratively with an adequately short notifies()
timeout.