Modify sequence state storage to eliminate dangling-pointer problem

Modify sequence state storage to eliminate dangling-pointer problem
exemplified by bug #671.  Moving the storage to relcache turned out to
be a bad idea because relcache might decide to discard the info.  Instead,
open and close the relcache entry on each sequence operation, and use
a record of the current XID to discover whether we already hold
AccessShareLock on the sequence.