Add support for entry counting in pgstats

Hi all,

One obstacle to the move of pg_stat_statements to the shmem pgstats
that we have in core is that there is no easy way to track the total
number of entries that are stored in the shared hash table of pgstats
for variable-sized stats kinds.

PGSS currently hash_get_num_entries() as a cheap way to get the number
of entries stored in its hash table, but moving PGSS to the in-core
stats facility means that we need a different approach to count these
entries.

I have looked at what can be done, and finished with a rather simple
approach, as we have only one code path when a new entry is inserted,
and one code path when an entry is removed when the refcount of an
entry reaches 0 (includes resets, drops for kind matches, etc.).  The
counters are stored in a uint64 atomic, one for each stats kind in
PgStat_ShmemControl, set only if the option is enabled.

I am wondering how much protection we want for the reads, and chose
the lightest "lossy" approach, leaving to stats kind what they want to
do when deallocating entries.  Hence, a stats kind may want to protect
the entry deallocations with an extra lock, but that's not required
either depending on how aggressive removals should happen.

Please find attached that adds an option for variable-sized stats
kinds given these the possibility to track the number of entries in
the shared hash table.  The option is named track_counts.  The patch
has some coverage added in the test module injection_points, in its
TAP test.

Thoughts are welcome.
--
Michael