httppeer: implement command executor for version 2 peer Now that we have a new API for issuing commands which is compatible with wire protocol version 2, we can start using it with wire protocol version 2. This commit replaces our hacky implementation of _call() with something a bit more robust based on the new command executor interface. We now have proper support for issuing multiple commands per HTTP request. Each HTTP request maintains its own client reactor. The implementation is similar to the one in the legacy wire protocol. We use a ThreadPoolExecutor for spinning up a thread to read the HTTP response in the background. This allows responses to resolve in any order. While not implemented on the server yet, a client could use concurrent.futures.as_completed() with a collection of futures and handle responses as they arrive from the server. The return value from issued commands is still a simple list of raw or decoded CBOR data. This is still super hacky. We will want a rich data type for representing command responses. But at least this commit gets us one step closer to a proper peer implementation. Differential Revision: https://phab.mercurial-scm.org/D3297