Now, the individual `ProcessQueueItem` functions decide whether to
acquire an `olock` or not instead of probing this from within the
worker loop. This is way easier than having to deal with the potential
out of order processing of items in the queue in both ways, i.e., we
don't want to send delete events for objects while their created events
haven't been processed yet and vice versa.
This commit restructures the queue items so that each one now has a method
`GetQueueLookupKey()` that is used to derive which elements of the queue are
considered to be equal. For this, there is a key extractor for the
`multi_index_container` that takes the `variant` from the queue item, calls
that method on it, and puts the result in a second variant type. The types in
that variant type are automatically deduced from the return types of the
individual methods.
As opposed to the previous version which used a complex data structure
to correctly manage the query priorities, this version uses two separate
queues for the high and normal priority writes. All high priority writes
are processed in FIFO order but over take all queries from the normal
priority queue. The later queue only be processed when the high priority
queue is empty.
