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)