nid and uid should match the column definitions in core

This patch updates the schema definition for the admin_message and admin_message_close tables so that the nid and uid columns match the definitions in the node and user tables. Specifically, this changes the nid and uid columns to be INT(10) instead of SMALLINT which allows creation and usage of admin_messages on drupal sites with nids/uids greater than the size of smallint.

Included is an update function admin_message_update_6001() to make the schema change for previously installed versions.

This has been tested locally and works when using a fresh install of the schema and when updating the schema.