enqueueing: improve when enqueueing many items w/ignore_duplicates enabled
enqueueing: improve when enqueueing many items w/ignore_duplicates enabled

When enqueueing many items having ignore_duplicates set to true we should only run a single SQL query to detect these duplicates. This commit also streamlines the execution path for single and multiple item use cases.

Note that this is a common usecase for Postqueue.
mediapeers/postqueue,mediapeers/postqueue