[req] nomad log, stop follow on allocation termination

### Nomad version
`Nomad v0.8.4 (dbee1d7d051619e90a809c23cf7e55750900742a)`

### Operating system and Environment details
OS X

### Issue
I use the `nomad alloc logs -f` a great deal. I would like seeing the behavior of terminating when the allocation is complete or no longer running. There will never be more logs, continuing (or following in general) an exited allocation doesn't seem to make much sense.

Thank you!

Start with the `nomad alloc logs -f` command and trace how follow mode handles allocation state. Check the existing behavior when an allocation terminates, then add coverage for stopping follow mode once the allocation is no longer running. Done means following exits automatically while logs for a running allocation continue to stream.

Written by the indexing model from the issue text.