This is a "more correct" API and we might as well fix this before it becomes
used all over the place. The new API returns ErrNoPublicKey when there is no
inlined key.
This *also* fixes a bug where the datastore-backed keystore would panic if
`ExtractPublicKey` returned `nil, nil`.
We switched to a slice to reduce the amount of memory the peerstore ended up
taking up unfortunately, this really killed us in allocations. Looking at
go-ipfs profiles, I'm worried that memory fragmentation is killing us so I'd
like to revert to the old behavior.
Note: The real solution here is dealing with "address abusers".