This applies lessons learned from the async batch DNS refresh action, in
particular making testing more robust.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=134833523
This allows handling of N asynchronous deletion requests simultaneously instead
of just 1. An accumulation pull queue is used for deletion requests, and the
async deletion [] is now fired off whenever that pull queue isn't empty,
and processes many tasks at once. This doesn't particularly take more time,
because the bulk of the cost of the async delete operation is simply iterating
over all DomainBases (which has to happen regardless of how many contacts and
hosts are being deleted).
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133169336
