Teach MockWebServer to throttle the request body.

We use a single throttle for both request and response. This is
sufficient for all of our current needs.

This breaks API compatibility with previous custom dispatchers.
Dispatchers that override peekSocketPolicy() must now override
peek() instead.
Merge pull request #663 from square/jwilson_0323_throttle_request_body

Teach MockWebServer to throttle the request body.