Implement support for notification namespaces.

[jmalloc]

Using the notation from the source code, the new message formats are as follows:

LISTEN: 'NL', // preamble, [namespaces]
UNLISTEN: 'NU', // preamble, [namespaces]

To be clear, namespaces is an array of strings, meaning that the header will be a single-element array containing another array.

• Both operations are idempotent, listening/unlistening to the same namespace multiple times has no effect and is not an error.
• Messages can be sent at any time.
• The only failure mode is a broker connection failure, in which case the websocket connection is closed anyway, so there is no message returned indicating a success (just like session create/destroy).

Additionally, the existing NOTIFICATION type gains namespace at the start of the header:

NOTIFICATION: 'NO', // preamble, [namespace, notificationType], payload

[jmalloc]

Blocked by rinq/httpd#11