Make swh.storage.archiver able to run in a separated db

In effect, this separates concerns between storing/reading contents and
archiving contents.

This updates:

• main schema to drop existing archiver table (070.sql)
• integration/unit tests
• Separate sql files to deal with install/upgrades in sql/archiver/ folder
• Add makefile to deal with local dump and restore actions

Remains to deal with updating when receiving new contents (as in the
original archiver's code)