spread not respected during node drain

It would be convenient if migrations (e.g., triggered by `nomad node drain` and governed by the `migrate` stanza) were able to spread new allocations.

This feature will be useful for us, since it will keep the `spread` configuration constraints even after draining nodes.

Start by tracing how `nomad node drain` triggers migrations governed by the `migrate` stanza, then inspect where allocation placement handles `spread` constraints. Done means migrations during node draining preserve the job's configured spread behavior for new allocations.

Written by the indexing model from the issue text.