Allow archiving deleted rows to shadow tables, for performance.

Implements bp db-archiving

Add versions of all tables with "shadow_" prepended to their names.
These shadow tables do not have indexes.

Adds API function archive_deleted_rows, which moves some deleted
rows from production tables to the corresponding shadow tables.
(It only does a few rows, to avoid blocking the process or hitting
the database too hard.  So it needs to be called regularly.)

Also adds a db archive_deleted_rows command to nova-manage.

Uses Boris Pavlovic's code for copying columns from existing tables
to new tables.

Change-Id: I7c8f2fb8794b6f9cd2531e54a1a9635010f8c97e
Merge "Allow archiving deleted rows to shadow tables, for performance."