cleanup Device Registration Endpoint and making 'addInstallation' async;...

NOTE: This needs to be rebased and merged _AFTER_ #371 got merged
Also, this commit is needed on both: `1.0.x` and `1.1.x`

Goal is to make the code behind the device registration endpoint async.

In theory we don't even need to return the JSON of the device/installation entity, but keeping that, for now.

Added unit tests for this as well