Upgrades trample extension-module data

When upgrading an extension, CRM_Core_Extensions::upgrade() performs an "uninstall" followed by an "install". This is reasonable for reports and payments processors (which don't generally create new data structures); however, for modules which must manage their own tables, this invokes hook_civicrm_uninstall and hook_civicrm_install which will cause 'DROP TABLE' and 'CREATE TABLE' – thereby destroying any data.

The upgrade process should instead call a new hook, hook_civicrm_upgrade.