Migrate PGPROC's backendLock into PGPROC itself, using a new tranche.

Robert Haas <rhaas@postgresql.org>

Commit: b319356f0e94a6482c726cf4af96597c211d8d6e
Author: Robert Haas <rhaas@postgresql.org>
Date: 2016-01-29T13:14:28Z
Releases: 9.6.0
Migrate PGPROC's backendLock into PGPROC itself, using a new tranche.

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

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