Add generic UC dropper

There is a lot of tables where we should add unique constraints
on upgrade and drop on downgrade, to avoid race conditions.
But there is a problem with SQLite. SQLite does not allow to drop unique constraints.
There is only one way to drop UC in SQLite:
1) create new table, with same columns and UC (but without UC that we want to drop)
2) copy data from old table
3) drop old table
4) create indexes in new table from old
5) rename new table to old table name

For other DB backends is used 'alter table'.

This patch add generic method for dropping UC.

blueprint db-unique-keys

Change-Id: I3e60578d3902559996943868abeb17b41153769b

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