peer: add an iterbatcher interface This is very much like ordinary batch(), but it will let me add a mode for batch where we have pathologically large requests which are then handled streamily. This will be a significant improvement for things like remotefilelog, which may want to request thousands of entities at once.

wireproto: document quirk of _callstream between http and ssh This tripped me up when trying to use it, so it feels like we should document this to avoid future pain.

peer: raise NotImplementedError for abstract submit() method Nothing should ever call this submit method directly as it should be overridden by concrete batcher implementations.

fsmonitor: new experimental extension Extension to plug into a Watchman daemon, speeding up hg status calls by relying on OS events to tell us what files have changed. Originally developed at https://bitbucket.org/facebook/hgwatchman