List all routes and lbtargets using gRPC

[PlagueCZ]

I added the ability to list all loadbalancer targets and all prefixes (incl. lbprefixes), in the same fashion as previously has been done with NATs, i.e. by specifying an "empty" identifier.

For prefixes, the drawback is that in the response, the VNI is not valid, it is the "empty" one.

• given the fact that target VNI is always the same as VNI anyway, this is a non-issue

For LB targets, the actual LB id is missing from the output.

• not ideal, but for checking whether both dpservices are orchestrated the same, this is enough.

Both of the above is easily fixable, but requires changes to the gRPC protocol, which I would rather not do. And given the fact that this already covers the intended purpose (quick check of HA dpservices), the need for the change is low in my opinion.

Fixes #745

[guvenc]

@PlagueCZ
This looks ok to me. Can we add a pytest test case where the loadbalancer behaviour is already tested, a call which would test the expected behaviour introduced here ?

[PlagueCZ]

Added pytest for all lists that have no "native" list call, i.e. use some special nul-value instead to force listing all entries.

[guvenc]

LGTM. Thanks for the tests as well.