420142 reimplemented graceful shutdown
420362 - Response/request listeners called too many times.

Wrapped on[Request|Response]XXX(XXXListener) listeners into their
specific interface so that they don't get notified multiple times.
Merge remote-tracking branch 'origin/master' into jetty-9.1

Conflicts:
	jetty-client/src/main/java/org/eclipse/jetty/client/HttpSender.java
	jetty-server/src/main/java/org/eclipse/jetty/server/handler/StatisticsHandler.java