Create exactly one instance of Left and Right handlers in MergingEventHandler
and make classes private. UPdate unit tests; also, some minor refactoring and
code cleanup.

P.S. MergingEventHandler is currently used only in Distributed Hash PCA
application which still uses old REEF API, so we'll have to change it
anyways.
use just left and right instead of rightHandler/leftHandler in MergingEventHandler
minor code cleanups in MergingEventHandler
Merge pull request #7 from Microsoft-CISL/sergiym_merging_event_handler

Disallow user to instantiate Left and Right handlers for MergingEventHandler