Add backend-level lock statistics
 
This commit adds per-backend lock statistics, providing the same
information as pg_stat_lock.  It is now possible to retrieve those stats
(lock wait counts, wait times, and fast-path exceeded count) on a
per-backend basis.
 
This data can be retrieved with a new system function called
pg_stat_get_backend_lock(), that returns one tuple per lock type based
on the PID provided in input.  Like pg_stat_get_backend_io(), this is
useful if joined with pg_stat_activity to get a live picture of the
locks behavior for each running backend.
 
pgstat_flush_backend() gains a new flag value, able to control the flush
of the lock stats.
 
This commit is straight-forward, relying on the infrastructure provided
by 9aea73fc61d4 (backend-level pgstats).
 
Bump catalog version.  No need to touch PGSTAT_FILE_FORMAT_ID as backend
statistics are never written to disk.
 
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Tatsuya Kawata <kawatatatsuya0913@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Tristan Partin <tristan@partin.io>
Reviewed-by: Rui Zhao <zhaorui126@gmail.com>
Discussion: https://postgr.es/m/aiAzEY+cMQb/W8yu@bdtpg