Support serving SVN repositories over SSH

Summary:
Ref T2230. The SVN protocol has a sensible protocol format with a good spec here:

http://svn.apache.org/repos/asf/subversion/trunk/subversion/libsvn_ra_svn/protocol

Particularly, compare this statement to the clown show that is the Mercurial wire protocol:

> It is possible to parse an item without knowing its type in advance.

WHAT A REASONABLE STATEMENT TO BE ABLE TO MAKE ABOUT A WIRE PROTOCOL

Although it makes substantially more sense than Mercurial, it's much heavier-weight than the Git or Mercurial protocols, since it isn't distributed.

It's also not possible to figure out if a request is a write request (or even which repository it is against) without proxying some of the protocol frames. Finally, several protocol commands embed repository URLs, and we need to reach into the protocol and translate them.

Test Plan: Ran various SVN commands over SSH (`svn log`, `svn up`, `svn commit`, etc).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7556