Introduces two new public gRPC endpoints (`Login` and `Logout`) and
includes refactoring of the equivalent net/rpc endpoints to enable the
majority of logic to be reused (i.e. by extracting the `Binder` and
`TokenWriter` types).
This contains the OSS portions of the following enterprise commits:
- 75fcdbfcfa6af21d7128cb2544829ead0b1df603
- bce14b714151af74a7f0110843d640204082630a
- cc508b70fbf58eda144d9af3d71bd0f483985893
These changes are necessary to ensure advertisement happens correctly even when datacenters are connected via network areas in Consul enterprise.
This also changes how we check if ACLs can be upgraded within the local datacenter. Previously we would iterate through all LAN members. Now we just use the ServerLookup type to iterate through all known servers in the DC.
