mutable files: expose version info to HTTP clients?

[warner]

in #328, we overhauled the internal API for mutable files, to make them safe to use for structured data that requires test-and-set semantics. But this is not currently exposed to the webapi: clients using HTTP can only blindly overwrite mutable files, making them unsuitable for carrying more structured data that must be manipulated outside the tahoe node.

I'm not convinced that passing a servermap outside (and back into!) the tahoe node over HTTP is a good idea. Maybe passing a handle to one might be safe.