add changes queue support

[benoitc]

Changes queues are the core of blobs synchronisations and can be used by indexers or other agents, They can are created on the source and are semi persistent.

Key mechanism is the following:

• anonymous changes queue are created by a POST to /<storage>/_queue. An id is returned allowing the listener to watch changes on it.
• named queue are created by a PUT to /storage/_queue/<name>. If the queue exists, a conflicts is returned (409) .
• Multiple listeners can watch a queue, the queue items are balanced between them. An option can be passed to only accept one listener for a queue.
• Queue can be watched using websockets or webrtc (data channels). A detection mechanism is provided to detect on the source the transport used.
• A watcher can continuously watch queue
• Items are received in bulk operation:
  1. During watching operations a controller ask for N blobs
  2. if no blobs are available a NOP is sent to the listener
  3. Once the N blobs are received by the listener an ACK is sent to the queue
  4. If an ACK is received on the queue, blobs received are deleted from the queue
  5. If a NOP operation is received on the queue, nothing is done. NOP operations are used to maintain the connection alive
• Queues are persistent.
• Queues can be automatically deleted if they has been active since a while. Optionnal.

[benoitc]

Queue backend can be pluggable as well imo. With a default to ETS saved from time to time to the disk. Thoughts?

[nrdufour]

It would be interesting to have a backend for queues or lists. And yes starting with an ETS or DETS ;)