net/http: use exponential backoff for polling in Server.Shutdown
Instead of always polling 500ms, start with an interval of 1ms and
exponentially back off to at most 500ms. 10% jitter is added to each
interval.
This makes Shutdown more responsive when connections and listeners
close quickly.
Also removes the need for the polling interval to be changed in tests
since if tests' connections and listeners close quickly Shutdown will
also return quickly.