[square] instrumenting the thread pool more systematically.
First pass to get rid of all the metrics persistence optimizer classes
	- Changed shouldPersist in AstyanaxWriter to use the logic in the StringMetricsPersistenceOptimizerClass
	- Added unit tests to test AstyanaxWriter for metrics persistence
More unit tests for the persistence functionality
Deleting all unnecessary classes
Configuring droppage of string metrics
Added documentation to the tests
Adding support for configuring the tenantids for whom string metrics need to be kept
Addressing review comments
Merge pull request #394 from tilogaat/clean_string_metrics_optimizer

Clean string metrics optimizer
Using getListProperty of the Configuration class
Merge pull request #395 from tilogaat/dropstringmetrics

Drop string metrics
Merge pull request #390 from jeeyoungk/jee.public.threadpool

Instrumenting threadpool more systematically
[square] refactoring the SlotKey

Removed the string `slotKey` that is used throughout the code, and replaced it with a class.
[square] Removing unused functions.

Now that `SlotKey` class exists, a lot of functions within `Granularity`
can be removed. Removed the locator range functionality that isn't being
used anywhere in the production code.
[square] adding tests.
[square] doesn't need to be private
[square] actually de-schedule itself.