pgsql: Add some optional code (conditionally compiled under #ifdef

Log Message:
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Add some optional code (conditionally compiled under #ifdef LWLOCK_STATS)
to track the number of LWLock acquisitions and the number of times we
block waiting for an LWLock, on a per-process basis. After having needed
this twice in the past few months, seems like it should go into CVS.

Modified Files:
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pgsql/src/backend/storage/lmgr:
lwlock.c (r1.38 -> r1.39)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lwlock.c.diff?r1=1.38&r2=1.39)