etcd: fix leaking fds

When etcd redirects to a leader - which happens every time fleet makes a request to a standby - it never closes the body of the response.

WARNING: This has not been tested, but it builds. We're waiting to see if this actually fixes the problem in production. Stay tuned.
@jonboulle 
There may be a race in `requestHTTP`.
If the request has been asked to be cancelled, no one would close the body of the response if the response is received before the request is actually cancelled.