…th driver.

There are 3 different issues that cause the same underlying problem: an executor
that the driver has killed during downscaling registers back with the block
manager in the driver, and the block manager from that point on keeps trying
to contact the dead executor.

The first one is that the heartbeat receiver was asking unknown executors to
re-register when receiving a heartbeat. That code path only really happens
when the executor dies because of a driver killing it, so there's no reason
to re-register.

The second one is a race between the heartbeat receiver and the DAG scheduler.
Both received notifications of an executor's addition and removal
asynchronously (the first one via the listener bus *and* an async local RPC,
the second via its own separate internal message queue). This led to
situations where they disagreed about which executors were really alive; the
change makes it so the heartbeat receiver is updated first, and once that's
done, then the DAG scheduler can update itself. This ensures the hearbeat
receiver knows which executors not to ask to re-register.

The third one is because the block manager couldn't differentiate between
an unknown executor (like one that's been removed) and an executor that needs
to re-register (like one the scheduler decided to unregister because of
too many fetch failures). The change adds code in the block manager master to
track which executors have been removed, so that instead of asking them to
re-register, it just ignores them.

While there I simplified the executor shutdown a bit since it was doing
some stuff unnecessarily.

Tested with existing unit tests, and by repeatedly runnins worklogs on k8s
with dynamic allocation; previously I'd hit these different issues somewhat
often, with the fixes I'm not able to reproduce them.