Avoid setup work for invalidation messages at start-of-(sub)xact.

 Avoid setup work for invalidation messages at start-of-(sub)xact.
 
 Instead of initializing a new TransInvalidationInfo for every
 transaction or subtransaction, we can just do it for those
 transactions or subtransactions that actually need to queue
 invalidation messages.  That also avoids needing to free those
 entries at the end of a transaction or subtransaction that does
 not generate any invalidation messages, which is by far the
 common case.
 
 Patch by me.  Review by Simon Riggs and Andres Freund.