Firebird 2.5.1 Sub-release

As well as a substantial number of bug fixes, this sub-release takes in a few minor improvements and optimizations that missed the initial release. In summary:

Significant for MacOSX 10.7 users was a bug that caused attempts to start Superserver and Superclassic to fail. The bug is described in Tracker ticket CORE-3589 and also in the Bug Fixes list for this sub-release.
An SQLSTATE context variable is now available in PSQL, parallel to the the context variables GDSCODE and SQLCODE that are used in WHEN blocks to test for error conditions.
A couple of improvements were made, that should help performance when global temporary tables (GTTs) are being used:
- The “undoing”, on rolling back the transaction, of changes made to GTTs that were created with the ON COMMIT DELETE ROWS option was an unnecessary overhead and has been bypassed.
- Garbage collection in global temporary tables was being delayed unnecessarily by active transactions in other attachments. That bottleneck is gone.
A needed optimization in the temporary space manager with regard to small chunk allocations has been implemented.
The Lock Manager has been provided with the capability to cancel waiting, avoiding a condition whereby a transaction in WAIT mode could wait interminably for the end of another transaction that could not be achieved by either a DELETE FROM MON$xxx or an fb_cancel_operation request.
The query optimizer now estimates the actual record compression ratio, enabling better guesses about the number of stored records in tables.
Some minor improvements to the remote interface:
- Any unused bytes of varchar values in the message buffer are now set to zero.
- Set the SO_KEEPALIVE option on the client TCP socket
The MON$STATEMENT_ID value constant is now kept constant among monitoring snapshots.
Ports of this sub-release were done for Linux/HPPA and Linux/alpha platforms.