wireproto: define frame to represent progress updates Today, a long-running operation on a server may run without any sign of progress on the client. This can lead to the conclusion that the server has hung or the connection has dropped. In fact, connections can and do time out due to inactivity. And a long-running server operation can result in the connection dropping prematurely because no data is being sent! While we're inventing the new wire protocol, let's provide a mechanism for communicating progress on potentially expensive server-side events. We introduce a new frame type that conveys "progress" updates. This frame type essentially holds the data required to formulate a ``ui.progress()`` call. We only define the frame right now. Implementing it will be a bit of work since there is no analog to progress frames in the existing wire protocol. We'll need to teach the ui object to write to the wire protocol, etc. The use of a CBOR map may seem wasteful, as this will encode key names in every frame. This *is* wasteful. However, maps are extensible. And the intent is to always use compression via streams. Compression will make the overhead negligible since repeated strings will be mostly eliminated over the wire. Differential Revision: https://phab.mercurial-scm.org/D2902