Move each SLRU's lwlocks to a separate tranche.

Robert Haas <rhaas@postgresql.org>

Commit: fe702a7b3f9f2bc5bf6d173166d7d55226af82c8
Author: Robert Haas <rhaas@postgresql.org>
Date: 2015-11-12T19:59:09Z
Releases: 9.6.0
Move each SLRU's lwlocks to a separate tranche.

This makes it significantly easier to identify these lwlocks in
LWLOCK_STATS or Trace_lwlocks output.  It's also arguably better
from a modularity standpoint, since lwlock.c no longer needs to
know anything about the LWLock needs of the higher-level SLRU
facility.

Ildus Kurbangaliev, reviewd by Álvaro Herrera and by me.

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