Work around Rails' shard swapping lock in the Solid Cache config

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.

config/cache.yml

@@ -1,17 +1,22 @@
-default: &default
+default_options: &default_options
   store_options:
     # Cap age of oldest cache entry to fulfill retention policies
     # max_age: <%= 60.days.to_i %>
     max_size: <%= 256.megabytes %>
     namespace: <%= Rails.env %>
 
+default_connection: &default_connection
+  connects_to:
+    database:
+      writing: :cache
+
 development:
-  database: cache
-  <<: *default
+  <<: *default_connection
+  <<: *default_options
 
 test:
-  <<: *default
+  <<: *default_options
 
 production:
-  database: cache
-  <<: *default
+  <<: *default_connection
+  <<: *default_options