Refactor ProxyHandler

[mhils]

This PR refactors ProxyHandler.read_request and its submethods to remove duplicate code and clearly separate request processing. As this is pretty much the core of mitmproxy, I'd love to have a second opinion on it. The github diff is pretty much useless, reading the new proxy.py lines 337-450 gives a better picture of the changes (old proxy.py here). If there are no objections, please merge.

By the way, this restructuring unfolded a few caveats, namely processing absolute-form request targets in reverse proxy or transparent mode. mitmproxy fails to do that correctly at the moment. In other words, this breaks a client -> transparent mitmproxy -> other proxy -> destination setup. However, that is a separate issue not introduced by these changes.

Cheers,
Max

[coveralls]

Coverage remained the same when pulling 21de99c on refactor_proxyhandler into 2be19a5 on master.

[cortesi]

This looks good to me. Lets merge, and see if anything breaks.