pgsql: A session that does not have any live snapshots does not have to

Log Message:
-----------
A session that does not have any live snapshots does not have to be waited for
when we are waiting for old snapshots to go away during a concurrent index
build. In particular, this rule lets us avoid waiting for
idle-in-transaction sessions.

This logic could be improved further if we had some way to wake up when
the session we are currently waiting for goes idle-in-transaction. However
that would be a significantly more complex/invasive patch, so it'll have to
wait for some other day.

Simon Riggs, with some improvements by Tom.

Modified Files:
--------------
pgsql/src/backend/commands:
indexcmds.c (r1.183 -> r1.184)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/indexcmds.c?r1=1.183&r2=1.184)
pgsql/src/backend/storage/ipc:
procarray.c (r1.48 -> r1.49)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.48&r2=1.49)
pgsql/src/include/storage:
lock.h (r1.115 -> r1.116)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lock.h?r1=1.115&r2=1.116)
procarray.h (r1.24 -> r1.25)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/procarray.h?r1=1.24&r2=1.25)