The initial implementation of global job stop for MRD looped over all the
regions in the CLI for expedience. This changeset includes the OSS parts of
moving this into the RPC layer so that API consumers don't have to implement
this logic themselves.
Adds a `-global` flag for stopping multiregion jobs in all regions at
once. Warn the user if they attempt to stop a multiregion job in a single
region.
Integration points for multiregion jobs to be registered in the enterprise
version of Nomad:
* hook in `Job.Register` for enterprise to send job to peer regions
* remove monitoring from `nomad job run` and `nomad job stop` for multiregion jobs
