Fix opcode transition from WAITLOCK to RUNNING

With the recent changes in the job queue, an old bug surfaced: we never
serialized the status change when in NotifyStart, thus a crash of the
master would have left the job queue oblivious to the fact that the job
was actually running.

In the previous implementation, queries against the job status were
using the in-memory object, so they 'saw' and reported correctly the
running status. But the new implementation just looks at the on-disk
version, and thus didn't see this transition.

The patch also moves NotifyStart to a decorator-based version (like the
other functions), which generates a lot of churn in the diff, sorry.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>