When lock is checked for conflict, ignore other locks on the record if
they wait for the requesting transaction.

lock_rec_has_to_wait_in_queue() iterates not all locks for
the page, but only the locks located before the waiting lock in the
queue. So there is some invariant - any lock in the queue can wait only
lock which is located before the waiting lock in the queue.

In the case when conflicting lock waits for the transaction of
requesting lock, we need to place the requesting lock before the waiting
lock in the queue to preserve the invariant. That is why we are looking
for the first waiting for requesting transation lock and place the new
lock just after the last granted requesting transaction lock before the
first waiting for requesting transaction lock.

Example:

trx1 waiting lock, trx1 granted lock, ..., trx2 lock - waiting for trx1
place new lock here -----------------^

There are also implicit locks which are lazily converted to explicit
ones, and we need to place the newly created explicit lock to the correct
place in a queue. All explicit locks converted from implicit ones are
placed just after the last non-waiting lock of the same transaction before
the first waiting for the transaction lock.

Code review and cleanup was made by Marko Mäkelä.