Dequeue one event per worker at a time

Each replication queue worker dequeues up to 10 events at a time and executes them sequentially. If the first job in the batch is slow, the remaining jobs end up blocked. This commit changes a queue worker to dequeue only one event at a time, leading to finer grained locking of events. This should allow for other free workers to pick up events that are not yet being handled.

Related to #2915 (closed)