Preserve commit timestamps across clean restart

An oversight in setting the boundaries of known commit timestamps during
startup caused old commit timestamps to become inaccessible after a
server restart.

Author and reporter: Julien Rouhaud
Review, test code: Craig Ringer

Author: alvherre

src/backend/access/transam/commit_ts.c

@@ -842,6 +842,8 @@ SetCommitTsLimit(TransactionId oldestXact, TransactionId newestXact)
 	else
 	{
 		Assert(ShmemVariableCache->newestCommitTsXid == InvalidTransactionId);
+		ShmemVariableCache->oldestCommitTsXid = oldestXact;
+		ShmemVariableCache->newestCommitTsXid = newestXact;
 	}
 	LWLockRelease(CommitTsLock);
 }