wireprotov2: define and use stream encoders Now that we have basic support for defining stream encoding, it is time to start doing something with it. We define various classes implementing stream encoders/decoders for the defined encoding profiles. This is relatively straightforward. We teach the inputstream and outputstream classes how to encode, decode, and flush data. We then teach the clientreactor how to filter received data through the inputstream decoder. One of the features of the framing format is that streams can span requests. This is a differentiating feature from say HTTP/2, which associates streams with requests. By allowing streams to span requests, we can reuse compression context data across requests/responses. But in order to do this, we need a mechanism to "flush" the encoder at logical boundaries so that receivers receive all data where it is expected. And a "flush" event is distinct from a "finish" event from the perspective of certain compressors because a "flush" will retain compression context state whereas a "finish" operation will not. This is why encoders have both a flush() and a finish() and each uses specific flushing semantics on the underlying compressor. The added tests verify various behavior of decoders via clientreactor. These tests do test some compression behavior via use of outputstream. But for all intents and purposes, server reactor support for encoding is not yet implemented. Differential Revision: https://phab.mercurial-scm.org/D4921