The region allocator marks freed regions "Condemned" before actually freeing them. Condemned regions are associated
with a transaction; before they can be reused / coalesced the transaction that condemned them must complete.
The region allocator used "TisActiveTransaction(xid)" to determine whether a given transaction had completed. The
problem is that Stasis now reuses transaction id's, and condemned regions were never reclaimed in practice. A transaction
fingerprint is a unique identifier (currently transaction id, rec_lsn) that can be used to distinguish transactions that
did not execute concurrently.
I considered an alternative design based upon end-of-transaction actions. This would have worked as well, but would
have been a more invasive change.
