Improve worker pool reservation to avoid starvation

[jnadler]

During Elasticsearch indexing, when a batch indexing error occurs we retry with exponential backoff. The worker pool checkout/checkin was managed outside of the block with the backoff sleep(). This caused a condition where if lots of bad documents are arriving and causing batch errors, lots of workers will be unavailable because they are doing backoff sleeps, and worker pool exhaustion is likely.

This change attempts to address that by moving the worker pool checkout/checkin closer to the batch indexing API invocation, which is appropriate as the worker pool is intended to bound the concurrency that Elasticsearch sees.