Allow setting the UseAsyn flag via a string to make it easier to set via spring/blueprint

Conflicts:
	rt/transports/http-netty/netty-client/src/main/java/org/apache/cxf/transport/http/netty/client/NettyHttpConduit.java
[CXF-5825] Fix the "verbose" logic in the tools
[CXF-5783] Fix client code generation
Recording .gitmergeinfo Changes