Fix Assert failure when a fastpath function call is attempted inside an

already-aborted transaction block.  GetSnapshotData throws an Assert if
not in a valid transaction; hence we mustn't attempt to set a snapshot
for the function until after checking for aborted transaction.  This is
harmless AFAICT if Asserts aren't enabled (GetSnapshotData will compute
a bogus snapshot, but it doesn't matter since HandleFunctionRequest will
throw an error shortly anywy).  Hence, not a major bug.

Along the way, add some ability to log fastpath calls when statement
logging is turned on.  This could probably stand to be improved further,
but not logging anything is clearly undesirable.

Backpatched as far as 8.0; bug doesn't exist before that.