Redundant unique constraint

federation extension have redundant unique constraint. The unique
constraint is totally useless, as (id, idp_id) columns are
columns of a composite primary key, which is unique by definition.

Change-Id: I93648ab89b26d7e77875a1faa8575a146224232f
Partial-Bug: #1292591
Migration DB_INIT_VERSION in common place

Init version value exists only in migration_helpers. migration_helpers
shouldn't know nothing about migrate repos, primarily because it is
intended for handling of any migrate repo in the project.

Change-Id: I8bceea0cf5eaf8e623efbb39e385b1990eca0a6f
Closes-Bug: #1308593
Merge "Redundant unique constraint"
Merge "Migration DB_INIT_VERSION in common place"