allow unset chunkSum

If chunkSum is not set we assume it has at least one chunk
Add the SA on stored result

The SA actually returns size now, so we pass this on.
At some point this should be refactored #266
Have TempStore set the size

cc @aldeed I just commuted it here :)
https://github.com/CollectionFS/Meteor-cfs-collection/commit/36a8ebe9878
62f866eb0c20317797daa62df3945#commitcomment-5924362