Allow subsequent calls to Start and Stop

[davseby]

Added a new flag to support calling stopped cache cleanup procedures as well as a mutex to control it. Now:

1. Start method will only run once even with the subsequent calls to it.
2. Stop will never block, in case the Start was not called or Stop was called before, it will return as the cache cleanup process is not running.

Closes #175

[coveralls]

Pull Request Test Coverage Report for Build 15237732703

Details

• 17 of 17 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 99.717%

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Totals
Change from base Build 15237488044:	0.006%
Covered Lines:	704
Relevant Lines:	706

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💛 - Coveralls