Implement HTTP APIs to get the list of client's in-flight and mqueue messages.

To get the first chunk of data:
 - GET /clients/{clientid}/mqueue_messages?limit=100
 - GET /clients/{clientid}/inflight_messages?limit=100

Alternatively:
 - GET /clients/{clientid}/mqueue_messages?limit=100&position=none
 - GET /clients/{clientid}/inflight_messages?limit=100&position=none

To get the next chunk of data:
 - GET /clients/{clientid}/mqueue_messages?limit=100&position={position}
 - GET /clients/{clientid}/inflight_messages?limit=100&position={position}

Where {position} is a value (opaque string token) of "meta.position" field from the previous response.

Mqueue messages are ordered according to their priority and queue (FIFO) order: from higher priority to lower priority.
By default, all messages in Mqueue have the same priority of 0.

In-flight messages are ordered by time at which they were inserted to the in-flight storage (from older to newer messages).