[bugfix] Fixes exceptional case in JobScheduler

CHANGELOG
=========
* Patched an exceptional case in `JobScheduler` that could cause
  `MesosJobFramework` to reach a non-sane and unrecoverable state.
* Corrected spelling in `TaskManagementResource` log message
* Converted erroneous `var` to `val` in `JobGraph`.
* Various light style fixes and cleanup.

/cc @florianleibert