Modify stream server to format data in json format and handle multiple builds.

Summary:
A client that connects to the stream server will always receive the output of
the last build.

Test Plan:
Have multiple client connect to the stream server "nc localhost 4343"
and start a build.

Reviewers: NicolasDP

Reviewed By: NicolasDP

Differential Revision: http://91.121.199.76/D29