Domain classes, stream/module deployments

Introduced Module and Stream domain classes. The stream/module
deployment flow is as follows:

1. Stream request is written to:

/xd/streams/<stream-name>

2. Admin "prepares" the stream by expanding out
the expected deployment:

/xd/streams/<stream-name>/source/<module-name>
                         /processor/
                         /sink/

3. Admin issues deployment request to specific
container:

/xd/deployments/<module-name>

4. The container server writes an ephemeral node
under the module that it just deployed under
the /xd/streams/.../<module-name> node.

5. This is an indication to the admin server
that the module was deployed.