Allow allow-core-timeout=off conf of PinnedDispatcher, see #2856
(cherry picked from commit 84933c123493a72ab95c6bf99c12748df675fa44)
Merge pull request #988 from akka/wip-2856-pinned-keep-alive-2.1-patriknw

Allow allow-core-timeout=off conf of PinnedDispatcher, see #2856