Overdue code review for transaction-level advisory locks patch.
Commit 62c7bd31 had assorted problems, most visibly that it broke PREPARE TRANSACTION in the presence of session-level advisory locks (which should be ignored by PREPARE), as per a recent complaint from Stephen Rees. More abstractly, the patch made the LockMethodData.transactional flag not merely useless but outright dangerous, because in point of fact that flag no longer tells you anything at all about whether a lock is held transactionally. This fix therefore removes that flag altogether. We now rely entirely on the convention already in use in lock.c that transactional lock holds must be owned by some ResourceOwner, while session holds are never so owned. Setting the locallock struct's owner link to NULL thus denotes a session hold, and there is no redundant marker for that. PREPARE TRANSACTION now works again when there are session-level advisory locks, and it is also able to transfer transactional advisory locks to the prepared transaction, but for implementation reasons it throws an error if we hold both types of lock on a single lockable object. Perhaps it will be worth improving that someday. Assorted other minor cleanup and documentation editing, as well. Back-patch to 9.1, except that in the 9.1 branch I did not remove the LockMethodData.transactional flag for fear of causing an ABI break for any external code that might be examining those structs.
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