432483 make osgi.serviceloader support for javax.servlet.ServletContainerInitializer optional
(cherry picked from commit 31043d25708edbea9ef31948093f4eaf2247919b)

Conflicts:
	jetty-annotations/pom.xml
Avoid duplicating existing headers when copying requests.
432528 - IllegalStateException when using DeferredContentProvider.

Fixed by using the correct idiom for calling the IteratingCallback.
Merged branch 'jetty-9.1.x' into 'master'.