Make some simple performance improvements in TransactionIdIsInProgress().

Make some simple performance improvements in TransactionIdIsInProgress().
For XIDs of our own transaction and subtransactions, it's cheaper to ask
TransactionIdIsCurrentTransactionId() than to look in shared memory.
Also, the xids[] work array is always the same size within any given
process, so malloc it just once instead of doing a palloc/pfree on every
call; aside from being faster this lets us get rid of some goto's, since
we no longer have any end-of-function pfree to do.  Both ideas by Heikki.