Extend framework from commit 53be0b1a to report latch waits.

WaitLatch, WaitLatchOrSocket, and WaitEventSetWait now taken an
additional wait_event_info parameter; legal values are defined in
pgstat.h.  This makes it possible to uniquely identify every point in
the core code where we are waiting for a latch; extensions can pass
WAIT_EXTENSION.

Because latches were the major wait primitive not previously covered
by this patch, it is now possible to see information in
pg_stat_activity on a large number of important wait events not
previously addressed, such as ClientRead, ClientWrite, and SyncRep.

Unfortunately, many of the wait events added by this patch will fail
to appear in pg_stat_activity because they're only used in background
processes which don't currently appear in pg_stat_activity.  We should
fix this either by creating a separate view for such information, or
else by deciding to include them in pg_stat_activity after all.

Michael Paquier and Robert Haas, reviewed by Alexander Korotkov and
Thomas Munro.