Because solid cache started using shard swapping when given a
`database:` key in https://github.com/rails/solid_cache/pull/219, and
Rails doesn't distinguish which database is being swapped and which
was locked.
Hopefully I can make a change upstream that will enable swap locking
only for a specific databases. Until then, let's just use a slightly
more verbose configuration to work around it.
David Heinemeier Hansson