Concurrency issues when setting multiple properties.

With the addition of atomic, multiple property updates there has been an increase in ConcurrentModificationExceptions. Part of this have been discovered with the work done in https://github.com/apache/accumulo/pull/2967.

This issue is to separate changes related to the CME problems from PR #2967. PR #2967, when merged, may stop the exceptions from being thrown, but there may be things that prevent the CMEs from occurring in the first place.

The underlying issue should be investigated and the logging and exception message can be improved, and these changes can take place independently of #2967 if not part of that PR.

The cause of the CMEs is unknown - but possibilities include concurrency issues with the tests - or interactions with the prop store cache.

Start by reviewing the changes and discussion in PR #2967, then investigate the ConcurrentModificationExceptions around the tests and property store cache mentioned in the issue. Compare concurrency behavior before and after the PR, and inspect the existing logging and exception messages. Done means identifying the underlying cause and making independently applicable improvements where needed.

Written by the indexing model from the issue text.