feat(chain)!: `TxGraph` contain anchors in one field
Previously, the field `TxGraph::anchors` existed because we assumed
there was use in having a partially-chronological order of transactions
there. Turns out it wasn't used at all.
This commit removes anchors from the `txs` field and reworks `anchors`
field to be a map of `txid -> set<anchors>`.
This is a breaking change since the signature of `all_anchors()` is
changed.
Update `TxGraph` field docs for `empty_outspends` and `empty_anchors`.