pull: make a single call to obsstore.add (issue5006) Prior to this, a pull of 90k markers (already known locally!) was making about 2000 calls to obsstore.add, which was repeatedly building a full set of known markers (in addition to other transaction overhead). This quadratic behavior accounted for about 50 seconds of a 70 second no-op pull. After this change, we're down to 20 seconds. While it would seem simplest to just cache the known set for obsstore.add, this would also introduce issues of correct cache invalidation. The extra pointless transaction overhead would also remain.