Add new table amos_snapshot

The table will hold the information about the most recent version of
each string at the given branch/language. The main benefit is expected
better response of the translator filter form.
Add a script for populating the new table with initial data
Update amos_snapshot on committing the stage
Use the new amos_snapshot table in the translator

This is expected to lead to better performance of the translator
interface. Obtaining mlang_component from snapshot is not affected as
there are no significant performance problems (yet? ;-)