Bugfix: instances of same go-plugin on different routes get mixed.
Bugfix: instances of same go-plugin on different routes get mixed.

When using declarative configuration, the cache object holding the plugin configs doesn't
seem to call the `load_plugin_from_db()` function.  (maybe it's preloaded some other way?)
This makes they never get the full DB key injected in the config, so the
`go.get_instance()` function falls back to using just the plugin name as the key to the
instance ID table.  Hilarity ensues.

This patch moves the injection of the __key__ and __seq__ fields from the
`load_plugin_from_db()` function to the outer `load_configuration()` function, so it works
on any DB strategy.

Kong/kong,Kong/kong,Kong/kong