CM discovery cache improvements

With KNOX-2680 in place, Knox maintains an in-memory cache that stores CM service discovery data. This repository was a huge improvement and solved some of the performance issues around CM service discovery, but there are some drawback:

• administrators are required to fine-tune the cache entry TTL as the default 10 minutes can be low in many cases
• the repository does not survive Knox restarts

I propose to implement the following changes:

• entries in that repository shall not be invalidated. They would live in memory throughout the entire lifetime of the Knox Gateway. The existing PollingConfigAnalyzer logic must be updated to keep that repository in synch.
• the repository should be serialized in the DATA_DIR during the shutdown of the Knox Gateway. It should also be populated from the previously persisted data at startup time. If there was a change on the cluster when Knox was down, PollingConfigurationAnalyzer will eventually sync the repository.