Reimplement DB collections for mirrors, repos and snapshots

See #765, #761

Collections were relying on keeping in-memory list of all the objects
for any kind of operation which doesn't scale well the number of
objects in the database.

With this rewrite, objects are loaded only on demand which might
be pessimization in some edge cases but should improve performance
and memory footprint signifcantly.

deb/local_test.go

@@ -124,6 +124,11 @@ func (s *LocalRepoCollectionSuite) TestByUUID(c *C) {
 
 	r, err := s.collection.ByUUID(repo.UUID)
 	c.Assert(err, IsNil)
+	c.Assert(r, Equals, repo)
+
+	collection := NewLocalRepoCollection(s.db)
+	r, err = collection.ByUUID(repo.UUID)
+	c.Assert(err, IsNil)
 	c.Assert(r.String(), Equals, repo.String())
 }