MINOR: event_hdl: add PAT_REF events

MINOR: event_hdl: add PAT_REF events

This is some prerequisite work for implementing PAT_REF events.

In this commit we define the PAT_REF event_hdl family (which gets family
slot id #2), with the following supported events:

  - EVENT_HDL_SUB_PAT_REF_ADD: element was added to the current version of
    the pattern ref
  - EVENT_HDL_SUB_PAT_REF_DEL: element was deleted from the current
    version of the pattern ref
  - EVENT_HDL_SUB_PAT_REF_SET: element was modified in the current version
    of the pattern ref
  - EVENT_HDL_SUB_PAT_REF_COMMIT: pending element(s) was/were commited in
    the current version of the pattern ref
  - EVENT_HDL_SUB_PAT_REF_CLEAR: all elements were cleared from the
    current version of the pattern ref

The goal is to be able to track a pat_ref struct in order to be notified
when it is updated. For performance reasons, events from this family won't
provide any additional info, and will only be published in the pat_ref
subscription list. Indeed, pat_ref may be updated at a relatively high
frequency (or worse, batch work), so we cannot afford doing expensive
treatment for each update.