Use tagsql.DB pointer as step database, to propagate changes
back and forth between actual database and migration.
Adds CreateDB operation to the migration step to be able to
create new dbs before executing migration action.
Adjusts storagenode database migration to use inner tagsql.DB
pointer of each database as step.DB.
Adjusts satellite dabase migration, adds proxy migrationDB field
to satellite db that wraps itself as tagsql.DB, pointer of which
is used as step.DB.
Change-Id: Ifed4de5b01a356cf7b37db64d2eaeb7b61982c5c
139a7ee959