Nomad server components that aren't in the `nomad` package like the deployment
watcher and volume watcher need to make RPC calls but can't import the Server
struct to do so because it creates a circular reference. These components have a
"shim" object that gets populated to pass a "static" handler that has no RPC
context.
Most RPC handlers are never used in this way, but during server setup we were
constructing a set of static handlers for most RPC endpoints anyways. This is
slightly wasteful but also confusing to developers who end up being encouraged
to just copy what was being done for previous RPCs.
This changeset includes the following refactorings:
* Remove the static handlers field on the server
* Instead construct just the specific static handlers we need to pass into the
deployment watcher and volume watcher.
* Remove the unnecessary static handler from heartbeater
* Update various tests to avoid needing the static endpoints and have them use a
endpoint constructed on the spot.
Follow-up work will examine whether we can remove the RPCs from deployment
watcher and volume watcher entirely, falling back to raft applies like node
drainer does currently.
