sql/pgq update

sql/pgq update

structure:
- event_seq field for pgq.tick table, to store last value
  from event_id_seq
- 'disabled' field for pgq.queue, which can disable any
  direct inserts to queue.  inserts are allowed only
  if session_role = 'replica'

functions:

- pgq.next_batch_info() as next_batch() but returns full details
- pgq.ticker(): now contains ticker logic that previously
  was located in python code - it uses the event_seq field
  in tick table to know about last sequence value
- pgq.insert_event_raw() - check disabled flag
- pgq.seq_getval() / pgq.seq_setval() functions for safe
  sequence variable manipulation

- remove denytriga, now regular triggers can play the role
- use OUT parameters instead of ret types
- pgq.force_tick(): silently ignore the request on disabled queues
- pgq.maint_retry_events(): fwd port bugfix from 2.1-stable
- pgq.maint_rotate_tables(): cleanup from 2.1-stable
- pgq.maint_tables_to_vacuum(): add more tables
- triggers/common.c: bugfix from 2.1-stable