DEV-31979: Introduce LLReqID, a class to help individual event API listeners

implement the ["reqid"] convention. This convention dictates that a response
LLSD from each such API should contain a ["reqid"] key whose value echoes the
["reqid"] value, if any, in the request LLSD.
Add LLReqID support to LLAresListener's "rewriteURI" service, LLSDMessage,
LLCapabilityListener and LLXMLRPCListener.