Avoid incrementing the CommandCounter when CommandCounterIncrement is called

 Avoid incrementing the CommandCounter when CommandCounterIncrement is called
 but no database changes have been made since the last CommandCounterIncrement.
 This should result in a significant improvement in the number of "commands"
 that can typically be performed within a transaction before hitting the 2^32
 CommandId size limit.  In particular this buys back (and more) the possible
 adverse consequences of my previous patch to fix plan caching behavior.
 
 The implementation requires tracking whether the current CommandCounter
 value has been "used" to mark any tuples.  CommandCounter values stored into
 snapshots are presumed not to be used for this purpose.  This requires some
 small executor changes, since the executor used to conflate the curcid of
 the snapshot it was using with the command ID to mark output tuples with.
 Separating these concepts allows some small simplifications in executor APIs.
 
 Something for the TODO list: look into having CommandCounterIncrement not do
 AcceptInvalidationMessages.  It seems fairly bogus to be doing it there,
 but exactly where to do it instead isn't clear, and I'm disinclined to mess
 with asynchronous behavior during late beta.