Nomad's original autopilot was importing from a private package in Consul. It
has been moved out to a shared library. Switch Nomad to use this library so that
we can eliminate the import of Consul, which is necessary to build Nomad ENT
with the current version of the Consul SDK. This also will let us pick up
autopilot improvements shared with Consul more easily.
Nomad inherited protocol version numbering configuration from Consul and
Serf, but unlike those projects Nomad has never used it. Nomad's
`protocol_version` has always been `1`.
While the code is effectively unused and therefore poses no runtime
risks to leave, I felt like removing it was best because:
1. Nomad's RPC subsystem has been able to evolve extensively without
needing to increment the version number.
2. Nomad's HTTP API has evolved extensively without increment
`API{Major,Minor}Version`. If we want to version the HTTP API in the
future, I doubt this is the mechanism we would choose.
3. The presence of the `server.protocol_version` configuration
parameter is confusing since `server.raft_protocol` *is* an important
parameter for operators to consider. Even more confusing is that
there is a distinct Serf protocol version which is included in `nomad
server members` output under the heading `Protocol`. `raft_protocol`
is the *only* protocol version relevant to Nomad developers and
operators. The other protocol versions are either deadcode or have
never changed (Serf).
4. If we were to need to version the RPC, HTTP API, or Serf protocols, I
don't think these configuration parameters and variables are the best
choice. If we come to that point we should choose a versioning scheme
based on the use case and modern best practices -- not this 6+ year
old dead code.
Reduce future confusion by introducing a minor version that is gossiped out
via the `mvn` Serf tag (Minor Version Number, `vsn` is already being used for
to communicate `Major Version Number`).
Background: hashicorp/consul/issues/1346#issuecomment-151663152
