Description:
------------
Add the ability to do commit retaining transactions using the Firebird PDO driver.  Commit retaining is faster when committing because it reuses server transaction resources.  The downside is you are still in the old transaction context so you won't see any new concurrent commits.  Commit retaining is ideally used when you have lots of small transactions that do not rely on other data that may be changed by another thread/user.

I have written a patch to add the ability to do commit retaining transactions using 
$dbh->setAttribute(PDO::FB_ATTR_COMMIT_RETAINING, true);
Any following commits or rollbacks will use their retaining equivalent after this attribute is set.  You can return to using normal commit and rollbacks by setting the attribute to false.
$dbh->setAttribute(PDO::FB_ATTR_COMMIT_RETAINING, false);