Light client needlessly re-fetch already Trusted blocks from its peer

[romac]

Moved over from #418

In the light client verification loop, we only look in the store for a block at the current height that is either Verified or Unverified (ie. we fetch it from a node), which may miss a block that has previously gone through fork detection and has been marked as Trusted. We thus have to fetch it again from the peer, which is wasteful.