* Implement a GetRouters RPC to RIS

This RPC returns all configured routers and their VRF IDs. This can be
used by RIS clients to discovery the routers. The router and the VRF ID
is a required parameter for all other RPCs.

* Update grpc to v1.28.0 to support protoc-gen-go v1.21.0

* Fix test

Co-authored-by: Oliver Herms <oliver.peter.herms@gmail.com>

* Replace ip/prefix map caches with btree caches.

* Cleanup

* Fix magic number

* fixed build

* init

* Refactoring

* Implementing basic daemon with configuration

* Add daemon

* Implement hot reloadable filters

* Adding more filter actions

* Make filters comparable

* Fix tests

* Fix tests

* Fix tests

* Cleanup

* Cleanup

* Cleanup

* Use documentation prefixes rfc5737

* Correct one prefix

* Fix copy&paste error

* added unique name for locRIBs

* added VRFs

* added error when no RIB configured in VRF

* vrf unique name ensurance

* fixed examples

* added missing test

* for convenience we provide a default RIB for v4 and v6 on each VRF

* removed debug output in main.go (api is coming)

* also removed debug output from fib example

* removed fib example

That's the way to go since Go 1.11 has been released.