code: func newMockBlockHeaderStore() headerfs.BlockHeaderStore {
  	return &mockBlockHeaderStore{
  		headers: make(map[chainhash.Hash]wire.BlockHeader),
  	}
  }
docs: // NewMockBlockHeaderStore returns a version of the BlockHeaderStore that's
  // backed by an in-memory map. This instance is meant to be used by callers
  // outside the package to unit test components that require a BlockHeaderStore
  // interface.