Ref T13507. For various messy reasons we can't blindly assume the server supports "gzip" -- but if the server tells us it does, we're on firmer ground.

If the server returns an "X-Conduit-Capabilities: gzip" header and we have compression support locally, compress subsequent requests.

This restores D21073, which was reverted by D21076.

With a gzip-asserting server, added debugging code and ran various "arc" commands. Saw the 2nd..Nth calls hit compression code.