make cxf consumer endpoints suspendable

Currently both CxfConsumer and CxfRsConsumer are SuspendableService, however they don't implements Suspendable interface, which means in DefaultShutdownStrategy, when catching kill signal, those consumer endpoints will be shutdown immediately. So even though inflight and pending exchanges are completed, they have no change to return to client since CXF Consumer endpoints already shutdown.

We should make cxf consumer endpoints suspendable to support graceful shutdown