Decoupled connection from handler

Now GerritHandler handles gerrit event and gerrit connection.
Even if you want to use event handle feature only, actual connection
is always needed.
To decrease network dependency, This patch isolates connection
features from GerritHandler.

In addition, the below features are provided by this patch.

* Enable to change nr. of workthreads during connection
* Refactor some parameters
* Use Executor instead of worker

We could do test without network connection. (So I keep existing tests so far.)