Support multiple ATs each connecting to multiple browsers each with multiple tabs

[aleventhal]

Each browser will spawn a broker (native messaging host + tcp client).
Each AT needs to inform all current/future brokers that they are listening/interested in js2at messages.
@zork

[aleventhal]

ATs could add a file to a well-known directory location when connect to a port. The native messaging host could discover these files and read the port number from there.

[aleventhal]

Seems like the multiple browsers (appId) and multiple tabs (docId) is addressed.
Need to support multiple ATs now.
While we do this, message broker not be launched if no AT connects to it.