A read or a replicated write to a volume server that is TCP-reachable but not
answering -- one still loading its volumes after a restart, or reached over a
stale keep-alive to a container that came back on a new IP -- blocked forever:
the shared HTTP transport had a dial timeout but no response timeout.
Add ResponseHeaderTimeout so a chunk read fails over to another replica and a
replicated write fails fast for the client to retry, and IdleConnTimeout so
pooled connections to a departed server are evicted instead of reused.
