Merge NFD-RIB into management thread
NFD-RIB currently interacts with FIB through control commands. After management is separated to its own thread (#4528), NFD-RIB can be merged into the same management thread, and interact with FIB through direct function calls.
This decreases the complexity of NFD-RIB and FIB management (
fib/remove-nexthop commands can be eliminated).