ensure mysql migrations run safely w/ multiple nodes

[pjenvey]

Although diesel's migrations run in a transaction, we're backed by mysql where DDL statements implicitly commit. Thus migrations could become problematic with multiple nodes deployed simultaneously.

We could likely lock the migration table separately during migrations. Maybe diesel migrations could do this for us.

[pjenvey]

I briefly discussed a diesel solution for this in their gitter, I'm not sure they'd take this on but I came to the conclusion it's doable via a mysql named lock (GET_LOCK). they aren't committed during transactions (which implicitly occur for most mysql DDL statements -- and thus why locking the migration table wouldn't work)