pgsql: Don't set recoveryLastXTime when replaying a checkpoint --- that

Log Message:
-----------
Don't set recoveryLastXTime when replaying a checkpoint --- that was a bogus
idea from the start since the variable is only meant to track commit/abort
events.  This patch reverts the logic around the variable to what it was in
8.4, except that the value is now kept in shared memory rather than a static
variable, so that it can be reported correctly by CreateRestartPoint (which is
executed in the bgwriter).

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xlog.c (r1.428 -> r1.429)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.428&r2=1.429)