pgsql-server: Add some code to Assert that when we release pin on a

Log Message:
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Add some code to Assert that when we release pin on a buffer, we are
not holding the buffer's cntx_lock or io_in_progress_lock. A recent
report from Litao Wu makes me wonder whether it is ever possible for
us to drop a buffer and forget to release its cntx_lock. The Assert
does not fire in the regression tests, but that proves little ...

Modified Files:
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pgsql-server/src/backend/storage/buffer:
bufmgr.c (r1.169 -> r1.170)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/buffer/bufmgr.c.diff?r1=1.169&r2=1.170)
pgsql-server/src/backend/storage/lmgr:
lwlock.c (r1.19 -> r1.20)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/lmgr/lwlock.c.diff?r1=1.19&r2=1.20)
pgsql-server/src/include/storage:
lwlock.h (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/lwlock.h.diff?r1=1.11&r2=1.12)