Fix handling of injection_points regarding pending stats

Hi all,

While doing more stuff related to pgstats and WAL, I have bumped on
the fact that the module injection_points uses for its
variable-numbered stats a pending flush callback but it does not
handle its entries so as these would be handled locally first, with a
final flush timed by pgstat_report_stat() that would itself call the
pending flush callback.  This is a less efficient practice, because it
requires to touch the pgstats dshash more often than necessary.

This is caused by the use of pgstat_get_entry_ref_locked() rather than
pgstat_prep_pending_entry() which would make sure to time the flush of
any pending entries appropriately depending on pgstat_report_stat().
I'd like to fix the module so as the flush timing is controlled like
all the other stats kinds, which is a better and more efficient
practice to have anyway as this code should stand as a template for
custom pgstats kinds.

While doing so, I have also noticed that injection_stats_flush_cb()
missed a pgstat_unlock_entry() when entries get locked.

All that is addressed in the patch attached.  Feel free if you have
any comments.

Thanks,
--
Michael