Previously, each PGPROC's backendLock was part of the main tranche,
and the PGPROC just contained a pointer.  Now, the actual LWLock is
part of the PGPROC.

As with previous, similar patches, this makes it significantly easier
to identify these lwlocks in LWLOCK_STATS or Trace_lwlocks output
and improves modularity.

Author: Ildus Kurbangaliev
Reviewed-by: Amit Kapila, Robert Haas