migrating schema with multiple connected storages results in a failed migration

[jameshclrk]

If a collection has multiple connected storages, for example with multiple replication handlers, there is a race condition where the underlying collection used for the migration is removed because the name is the same (see sqlite-trial error).

I haven't tried all storages, but I have seen it on memory and sqlite-trial.

see #7008

[pubkey]

Fixed here: #7036