Implement Initial Chain Reader Trait

[dutterbutter]

The TipIndex methods should be accessible via a ChainReader trait. Below demonstrates what that might look like.

• ChainReader trait

// ChainReader reads the chain store.

trait ChainReader {
  GetHead() -> TipSetKey

  GetTipSet(tsKey TipSetKey) -> TipSet

  GetTipSetStateRoot(tsKey TipSetKey) -> cid.Cid

  GetTipSetReceiptsRoot(tsKey TipSetKey) ->  cid.Cid

  HasTipSetAndState(tsKey TipSetKey) -> bool

  HasTipSetAndStatesWithParentsAndEpoch(pTsKey TipSetKey, epoch ChainEpoch) -> bool

  GetTipSetAndStatesByParentsAndEpoch(pTsKey TipSetKey, epoch ChainEpoch) -> []TipSetMetadata

  PutTipSetMetadata(tsas TipSetMetadata)
}

The above methods exist in tip_index.go.