rework hash

[jwoodmanbuildium]

@jeremydmiller - I've reworked the initial PR to handle #398 with some additional changes after the feedback/revert.

• Removed the conditional logic on the hot paths by adding an InstanceMapFactory. This new class will generate a data structure specific implementation of InstanceMap. This avoids all of the conditional checks when retrieving/updating the hashes.
• Fixed the breaking tests - this was just an updated constructor that was not calling the base constructor appropriately.
• Added a static property to define the default InstanceMap behavior. This uses the ImHashMap by default, but can be updated in code or through an environment variable (added the latter option so tests can be executed against both behaviors).