Use condition handlers to immediately rollback on lock timeouts inside the stored proc

[jamadden]

If we'd already taken some locks and then later get a lock error, only the statement that got the error is rolled back (I think this only applies to the readcurrent case). The other locks are kept until the transaction is rolled back. But if the client is busy, that could take some time. We should catch and handle that in the proc and immediately roll back.

MySQL condition handlers can't do much so the code will be ugly, and we'll have to explicitly SIGNAL to "reraise" the exception, I think.

See also #310.

[jamadden]

The final commit lock effectively isn't allowed to timeout, so this only applies to the object locks.