watchman: teach watchman to talk to EdenFS on Windows
watchman: teach watchman to talk to EdenFS on Windows

Summary:
This still requires support from EdenFS in order to do much
of use, but it takes us a step closer:

* Pull in cpptoml when building with Eden support
* On Windows, when we locate the `.eden` directory, load and parse
  the config file in order to determine the socket path
* If the EdenView constructor throws, treat it as a terminal error
  so that we don't fallback to the regular filesystem watcher.
  This is important because current EdenFS builds don't implement
  the journal thrift API endpoint yet.

Reviewed By: pkaush

Differential Revision: D20504752

fbshipit-source-id: 48bbad49f1641698aa7d7b85674e3ddf4d4e617d

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst