Perform lease synchronization in transaction

In this MR, the Job class is completely killed in favor of doing everything in the datastore/scheduler.

One goal of these changes is to not heavily change the existing behavior, but to ensure that almost all changes made are being performed in a single transaction. I compared these findings to the Remote Workers API spec and validated if the behavior makes sense.

Question: In the MR I removed some function which used to produce metrics. Notably the update_job and update_lease methods are removed. We're the metrics provided by these generic update methods of any value, or should I look to add them back into various functions?