Improve polling in segment allocation queue (apache#15590)

Description
When batchAllocationWaitTime is set to 0, the segment allocation queue is polled continuously even when it is empty. This would take up cpu cycles unnecessarily.

Some existing race conditions would also become more frequent when the batchAllocationWaitTime is 0. This PR tries to better address those race conditions as well.

Changes
Do not reschedule a poll if queue is empty
When a new batch is added to queue, schedule a poll
Simplify keyToBatch map
Handle race conditions better
As soon as a batch starts getting processed, do not add any more requests to it