[PATCH 0 of 4 V2] Improve filelog extensibility

[Durham Goode]

V2:
Remove unnecessary hooks (patches 1 & 2 from before). Change wireproto.stream to not have an optional argument. Instead I moved the file stream walk to a separate function so extensions can just override that. Removed old patches 4 and 5 since they are already queued upstream.

V1:
I'm working on an extension that will keep all file contents remotely on the server (to improve performance on large repos).  This series adds hooks and modifications to core mercurial to enable the extension to better manipulate filelog logic in core mercurial.