446563 - Null HttpChannel.getCurrentHttpChannel() in ServletHandler.doFilter().

Moved the finally block that resets the current HttpChannel and the
thread name after the handling of the COMPLETE action.
This is necessary because AsyncListener.onComplete() (triggered by
the handling of the COMPLETE action) may perform a
RequestDispatcher.forward() that requires the current HttpChannel
thread local to be set.
Merged branch 'jetty-9.2.x' into 'master'.