Rename TransactionChain functions

We call this thing a "transaction block" everywhere except in a few
functions, where it is mysteriously called a "transaction chain".  In
the SQL standard, a transaction chain is something different.  So rename
these functions to match the common terminology.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>

src/backend/commands/discard.c

@@ -63,7 +63,7 @@ DiscardAll(bool isTopLevel)
 	 * DISCARD ALL inside a transaction block would leave the transaction
 	 * still uncommitted.
 	 */
-	PreventTransactionChain(isTopLevel, "DISCARD ALL");
+	PreventInTransactionBlock(isTopLevel, "DISCARD ALL");
 
 	/* Closing portals might run user-defined code, so do that first. */
 	PortalHashTableDeleteAll();