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.
Per my previous comment, it should already be closed. `exhaust()` --> `one()` --> `requestFunc` (== `requestHttp`). `requestHTTP` should always close the body before the response is returned.