Multiple databases support

[wata727]

See also #12
See also #13

Currently this gem doesn't support multiple databases and you have to explicitly call TenantLevelSecurity.switch! when connecting other databases. But this is obviously inconvenient in read replicas.

One possible solution is that TenantLevelSecurity.switch! switches all connections on the current thread. This can be implemented by referring to AcitveRecord::Base.clear_query_caches_for_current_thread. See also rails/rails#35089
Note that some versions have different ways of getting the connection pool.

Another solution is to add a hook that fires when connecting to another database and run TenantLevelSecurity.switch! there. This is close to the current design of switching tenants using checkout callbacks, but I'm not sure if the same way works for multiple databases.