Move advisory locks to own connection handler. - #39758

Removes the use of `ActiveRecord::AdvisoryLockBase` since it inherits
from `ActiveRecord::Base` and hence share module attributes that are defined in `ActiveRecord::Base`.
This is problematic because establishing connections through
`ActiveRecord::AdvisoryLockBase` can end up changing state of the default
connection handler of `ActiveRecord::Base` leading to unexpected
behaviors in a Rails application.

In the case of rails#39157,

Running migrations with `rails db:migrate:primary_shard_one` was not working as
the application itself defined the following

```
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to shards: {
    default: { writing: :primary },
    shard_one: { writing: :primary_shard_one }
  }
end
```

In the database migrate rake task, the default connection was
established with the database config of `primary_shard_one`. However,
the default connection was altered to that of `primary` because
`ActiveRecord::AdvisoryLockBase.establish_connection` ended up loading
`ApplicationRecord` which calls `connects_to shards:`. Since all we
really need here is just a normal database connection, we can avoid
accidentally altering the default connection handler state during the migration
by creating a custom connection handler used for retrieving a connection.