The `ephemeral_disk` block's `migrate` field allows for best-effort migration of
the ephemeral disk data to new nodes. The documentation says the `migrate` field
is only respected if `sticky=true`, but in fact if client ACLs are not set the
data is migrated even if `sticky=false`.
The existing behavior when client ACLs are disabled has existed since the early
implementation, so "fixing" that case now would silently break backwards
compatibility. Additionally, having `migrate` not imply `sticky` seems
nonsensical: it suggests that if we place on a new node we migrate the data but
if we place on the same node, we throw the data away!
Update so that `migrate=true` implies `sticky=true` as follows:
* The failure mode when client ACLs are enabled comes from the server not passing
along a migration token. Update the server so that the server provides a
migration token whenever `migrate=true` and not just when `sticky=true` too.
* Update the scheduler so that `migrate` implies `sticky`.
* Update the client so that we check for `migrate || sticky` where appropriate.
* Refactor the E2E tests to move them off the old framework and make the intention
of the test more clear.
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