repo: Don't require a txn for writing

The current "transaction" symlink was introduced to fix issues with
interrupted pulls; normally we assume that if we have a metadata
object, we also have all objects to which it refers.

There used to be a "summary" which had all the available refs, but I
deleted it because it wasn't really used, and was still racy despite
the transaction bits.

We still want the pull process to use the transaction link, so don't
delete the APIs, just relax the restriction on object writing, and
introduce a new ostree_repo_set_ref_immediate().