Improve ReliableProxy resend across a slow link, see #2849

* The within margin was too small. On my machine the transition to Idle is done
  when 400 ms is remaining, which is too timing sensitive.
* Improved the "resend across a slow link" test to actually trigger resending,
  which it didn't do before.
Merge pull request #998 from akka/wip-2849-ReliableProxySpec-slow-link-patriknw

Improve ReliableProxy resend across a slow link, see #2849