Store sessions event times separately
Store sessions event times separately

Store all event time stamps in the new `Session.times` dict.
(Note: eventually these assignments may be moved entirely to the relevant
measurement app).

Adjust call popping logic (it seems sometimes FS will mysteriously change
the `call_uuid`, when under load, for a very small % of sessions) to fall
back to the value received in the first event for a session instead of
always relying on the channel var's current value. Also, only pop the
call when it contains no more sessions not when the session that initiated
the call is hungup.

Move the `get_pool` factory here from `switchy.apps.call_gen.py`

sangoma/switchy