For googlecloudplatform/gcsfuse#27.

For googlecloudplatform/gcsfuse#27.

To help investigate googlecloudplatform/gcsfuse#27.

Fixes #6.

The free function is more flexible, including being able to be used from
outside the daemon process.

And provide a helper to make it convenient to do so.

Having the server implementation call Respond makes it unnecessarily tricky to
implement FileSystem methods that continue computations in the background and
return immediately, for example enqueuing writes/flushes for a worker
goroutine.

This aovids the fundamental race in #3.