Ensure we are not exhausting the sqlalchemy pool

Because of eventlet, notification messages can be consumed more quickly
that than they are wroted to the database, this have to effect to exhaust
the sqlalchemy connection pool and raise:

TimeoutError: QueuePool limit of size 5 overflow 10 reached, connection
timed out, timeout 30

The patch creates a eventlet pool to ensure that the pool never exhausts
and prints a warning message in the log to inform that perhaps the pool
size need to be increased.

Fixes bug #1243251

Change-Id: I5ccdc141c441db32fab361783a31a96015516878
Add Event API

This commit adds v2/event_types, v2/events and
v2/events/*/traits. Currently, the event api is
limited to admin users, but is configurable in
the policy.json file. All users that are able to
access the event api can view all events in the
system, it is not restricted per project id.
Operators other than 'eq' have not yet been
implemented.

This change re-names the parameters in the
event_filter to class so they are more
end-user friendly.

implements bp specify-event-api

Change-Id: I0ccd4ef81cac5f8f57a450dc980ea85ff8fe89ec
Merge "Add Event API"
Merge "Ensure we are not exhausting the sqlalchemy pool"