[RigoDaemon] drop sleep()

With the new locking strategy, this is no longer necessary
[entropy.locks] UpdatesNotificationResourceLock: do not dictate any particular lock mode
[solo.commands.install] acquire the UpdatesNotificationResourceLock in shared mode

This will temporarily disable available updates notifications from
RigoDaemon
[RigoDaemon] use UpdatesNotificationResourceLock to avoid signal spam

If this resource lock is acquired, block until the lock is released
so that we will not keep sending status updates to the connected clients