Temporary pins when adding looks buggy

First, we don't appear to actually mark them as temporary. The following code appears dead in practice:

https://github.com/ipfs/go-ipfs/blob/1a0d6ec2ba1e114293cbb0a7d2394d7b48a554c2/core/coreunix/add.go#L198-L204

Second, this is *not* crash safe. We need a way to add in-memory pins.