migration_manager: Get database through storage_proxy
migration_manager: Get database through storage_proxy

There are several places where migration_manager needs storage_service
reference to get the database from, thus forming the mutual dependency
between them. This is the simplest case where the migration_manager
link to the storage_service can be cut -- the databse reference can be
obtained from storage_proxy instead.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla