Only create circulation_events on circulation managers

[courte]

Because CirculationEvents are tied in with core actions, they're being calculated on the metadata wrangler, resulting in lots of records and slower migration times. It would be nice to make event tracking optional outside of the circulation manager.

These are the types of events that are currently being tracked:

metadata=> select distinct type, count(id) from circulationevents group by type;
              type               | count
---------------------------------+--------
 distributor_availability_notify |  33286
 distributor_license_add         |  74282
 distributor_title_remove        | 109046
 distributor_check_out           | 985066
 distributor_title_add           | 392437
 distributor_license_remove      |  14224
 distributor_check_in            | 989866
 distributor_hold_place          | 650488
 distributor_hold_release        | 339769
(9 rows)

A brief glance suggests this might be happening in metadata_layer.py:CirculationData#license_pool and model.py:LicensePool#update_availability.