Prior to this commit, there is a race condition in the Feature Migration
Status API where the returned status can be `MIGRATION_NEEDED`, even if
a migration is already in progress (and therefore the returned value
should have been `IN_PROGRESS`). This commit adds a test for this case
which reliably fails without the fix, and fixes the bug.

The fix is straightforward: While we already examine the persistent task
metadata to determine progress, the part of that metadata that we
examined did was not updated until the task's been running for a bit.
However, if we check for the *existence* of the task metadata, that is
guaranteed to be in the cluster state by the time the request to start the
migration completes (and is removed immediately after the task finishes
- that's why we have separate metadata for the migration results instead
of just using the task state).