AMQ-7050: Allow alternate persistence mechanism with SubQueueSelectorCacheBrokerPlugin

[alechenninger]

More details in JIRA. Had a go at this by pulling out an interface for cache interactions and including default, File-based implementation that behaves just as before.

This flexibility is designed to allow future, alternative implementations such as a JDBC-based cache (e.g. for folks using JDBC persistence).

Thanks for review!

[sheri21]

https://github.com/apache/activemq/pull/297#issue-214146120

[jbonofre]

@alechenninger do you have time to refactore this PR as a complete new plugin or do you want me to do it ? Thanks !

[ehossack-aws]

@alechenninger do you have time to refactore this PR as a complete new plugin or do you want me to do it ? Thanks !

@jbonofre @alechenninger Since this is an old PR and your availability might have changed, I can offer to also refactor this PR to help move it forward.

[alechenninger]

@jbonofre @ehossack-aws I'm available, I just commented above. Trying to better understand what we're really after with the refactoring recommendation before moving forward.

[jbonofre]

I just did a new pass. I agree with the change however, this PR needs:

1. fix the missing ASF headers in couple of files
2. rebase on master I will do a new final pass and merge as soon as it's OK.

[jbonofre]

Non active PR