Potential S3 resource leaks exposed by 5.1

[qqmyers]

The switch in v5.1 to use a single AWS client appears to have exposed some additional resource leaks /open connections to S3. One source of these may be some uses of Java Channels where the Channel is not closed. This issue is a placeholder for work to fix those leaks, check for others, and potentially to add some monitoring/retry capabilities to avoid situations where S3 connections fail (causing downloads as well as display of thumbnails, etc. to fail).
(Note #7308 increases the default pool size and makes it configurable. Setting the pool size to a larger value should help.)