In various places we update the pool size to use for the database, then
reconnect. The global instance of
Gitlab::Database::LoadBalancing::Configuration is memoized, and may be
read before the pool size is updated. This can result in the pool size
being out of date, if database.yml uses a value different from the one
GitLab wants.

To work around this, LoadBalancing::Configuration#pool_size now always
reads the value from the underlying model. All other LB settings are
still persisted in-memory, as we don't support changing these at runtime
anyway.

Changelog: fixed