pgsql: Modify snapshot definition so that lazy vacuums are ignored by

Log Message:
-----------
Modify snapshot definition so that lazy vacuums are ignored by other
vacuums. This allows a OLTP-like system with big tables to continue
regular vacuuming on small-but-frequently-updated tables while the
big tables are being vacuumed.

Original patch from Hannu Krossing, rewritten by Tom Lane and updated
by me.

Modified Files:
--------------
pgsql/src/backend/access/transam:
twophase.c (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.21&r2=1.22)
xact.c (r1.224 -> r1.225)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.224&r2=1.225)
xlog.c (r1.244 -> r1.245)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.244&r2=1.245)
pgsql/src/backend/catalog:
index.c (r1.269 -> r1.270)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c.diff?r1=1.269&r2=1.270)
pgsql/src/backend/commands:
vacuum.c (r1.335 -> r1.336)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.335&r2=1.336)
pgsql/src/backend/storage/ipc:
procarray.c (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c.diff?r1=1.14&r2=1.15)
pgsql/src/backend/storage/lmgr:
proc.c (r1.178 -> r1.179)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c.diff?r1=1.178&r2=1.179)
pgsql/src/include/storage:
proc.h (r1.89 -> r1.90)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/proc.h.diff?r1=1.89&r2=1.90)
procarray.h (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/procarray.h.diff?r1=1.9&r2=1.10)