Copy GerritHandler to GerritConnection

For preparation, simple copy.
Isolate connection feature

* Rename class to GerritConnection in duplicated GerritConnection.java
* Remove unnecessary code in GerritConnection and GerritHandler
* Coodinate StreamWatchdog parameter
Add Connector interface

Coordinator interface cannot be implemented to GerritConnection
since there is no workqueue. So add Connector interface tnen move
reconnection feature to it.
Implement Connection/Handler solution to plugin

gerrithudsontrigger follows new Connection/Handler solution in
gerrit-events. The feature for saving and restoring lisners is
removed since the lifetime of handler equals the ones for plugin.
Add new test class

Add new test class for GerritConnection
Fix tests

Coordinate some tests following new Connection/Handler solution
Merge pull request #64 from rinrinne/connection-isolation-2

Isolate connection