Fix volatility marking of commit timestamp functions

They are marked stable, but since they act on instantaneous state and it
is possible to consult state of transactions as they commit, the results
could change mid-query.  They need to be marked volatile, and this
commit does so.

There would normally be a catversion bump here, but this is so much a
niche feature and I don't believe there's real damage from the incorrect
marking, that I refrained.

Backpatch to 9.5, where commit timestamps where introduced.

Per note from Fujii Masao.

src/include/catalog/pg_proc.h

@@ -3082,10 +3082,10 @@ DESCR("view two-phase transactions");
 DATA(insert OID = 3819 (  pg_get_multixact_members PGNSP PGUID 12 1 1000 0 0 f f f f t t v 1 0 2249 "28" "{28,28,25}" "{i,o,o}" "{multixid,xid,mode}" _null_ _null_ pg_get_multixact_members _null_ _null_ _null_ ));
 DESCR("view members of a multixactid");
 
-DATA(insert OID = 3581 ( pg_xact_commit_timestamp PGNSP PGUID 12 1 0 0 0 f f f f t f s 1 0 1184 "28" _null_ _null_ _null_ _null_ _null_ pg_xact_commit_timestamp _null_ _null_ _null_ ));
+DATA(insert OID = 3581 ( pg_xact_commit_timestamp PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 1184 "28" _null_ _null_ _null_ _null_ _null_ pg_xact_commit_timestamp _null_ _null_ _null_ ));
 DESCR("get commit timestamp of a transaction");
 
-DATA(insert OID = 3583 ( pg_last_committed_xact PGNSP PGUID 12 1 0 0 0 f f f f t f s 0 0 2249 "" "{28,1184}" "{o,o}" "{xid,timestamp}" _null_ _null_ pg_last_committed_xact _null_ _null_ _null_ ));
+DATA(insert OID = 3583 ( pg_last_committed_xact PGNSP PGUID 12 1 0 0 0 f f f f t f v 0 0 2249 "" "{28,1184}" "{o,o}" "{xid,timestamp}" _null_ _null_ pg_last_committed_xact _null_ _null_ _null_ ));
 DESCR("get transaction Id and commit timestamp of latest transaction commit");
 
 DATA(insert OID = 3537 (  pg_describe_object		PGNSP PGUID 12 1 0 0 0 f f f f t f s 3 0 25 "26 26 23" _null_ _null_ _null_ _null_ _null_ pg_describe_object _null_ _null_ _null_ ));