The `DBMasterPos` class is not JSON-serializable, so
we can not transfer the job in the kafka-based queue.
Before, we were waiting for slaves before executing the
job till the point in time when the job was submitted,
now we will just wait for slaves till the point in time
the job was executed. That lets up not to include the
database master position in the event and make it serializable.
Bug: T192945
Change-Id: I7c754bd1e899bad030cc6434be19daf2542e015f
NotificationJob used to include an instance
of EchoEvent, which is not serializable.
With this change, it only includes the
event Id (int) and retrieve the event
instance in its run() function.
Bug: T192945
Change-Id: I00950ddfa37717c7dfc19efdca9701693622da5d
