Add HTTP queue mechanism to libmq

[nevali]

libmq would benefit from support for queues which are exposed via HTTP:

POST /queue-uri
Adds a new item to a queue

GET /queue-uri[?wait=SECONDS]
Pops an item from the queue, optionally waiting for new items to appear

HTTP headers are used to convey message attributes.

While AMQP queues can be exposed using this interface, this also allows for other kinds of message queue to be used instead.

Note: GET should probably not be used to retrieve an item, because it should be idempotent; perhaps it should be a combination of GET and DELETE together, passing an instance identifier in the request parameters to enable uniqueness and filtering.

[nevali]

Closing; this can be implemented in bbcarchdev/libmq if required.