Fix migration that breaks Grenade jobs

Migration 33c3db036fe4_set_length_of_description_field_metering
needs to be changed as it is a reason of Grenade job's fail.

Closes-bug: #1307344

Change-Id: Ia57d9a60847e4a34977e40fb8b09181bdbd9c39b
Replace loopingcall in notifier with a delayed send

The loopingcall thread here was started before processes fork and so
the thread stops working after the fork call.  This is a problem that
will probably need to be worked out in the long run.

To ensure that this notifier works correctly in all processes, this
change replaces the persistent loopingcall thread with a thread
created on demand to delay and batch up notifications.  The first
notification will trigger spawning the thread to wait to send it.  Any
notifications that come in the meantime will notice that there is
already a thread waiting to send and will return without spawning.

Change-Id: I519d4e89b8cee341c0e1cfffbce3e77151e8202a
Closes-Bug: #1301035
Merge "Replace loopingcall in notifier with a delayed send"
Merge "Fix migration that breaks Grenade jobs"