[improvement] show migration progress in API

### Nomad version
nomad 0.7.1

I'm using nomad for some stateful services with clustering support (galera and mongodb). All data is stored inside /local directory, but when i need to drain node i'm getting a long migration process. The problem is that i don't understand current migration status. I think you should set migration progress somewhere in API in DesiredDescription, so it will looks so :"alloc is being migrated, 1MB of 1234MB transferred, 0.1%"

Start by tracing the API representation mentioned in DesiredDescription and the migration status exposed while draining a node. Determine which migration measurements are available for /local data, then define how transferred and total bytes should appear in the API; done means a draining allocation reports meaningful progress.

Written by the indexing model from the issue text.