Refactor MessageTracker

Refactor MessageTracker

This patch updates MessageTracker implementation with the following:
- lower overhead when dealing with StopWatch under normal conditions
- prevent Context from being instantiated from the outside
- use a Ticker instance for reliable testing
- use Stopwatch.isRunning() instead of explicit done/enable boolean
- properly reset interval timer when expected message is received
- add explicit @NotThreadSafe and @Beta annotations
- move currentMessage to CurrentMessageContext

Change-Id: Idd553a39a8fb885a5c05e391cb4ae45384a59f07
Signed-off-by: Robert Varga <rovarga@cisco.com>