libflux: rework message api

Add a protocol flag to indicate presense of routing delim,
and accessors to get/set a flag indicating its presense,
and accessors for the route frame stack.

Add accessors for payload and topic.

Add unit tests.
libutil: rework request functions

Rework API and internals to use new message API.
api: use new request/message API
broker: use new message/request API

Only the minimum changes to ensure rte flags are set
properly, etc..