Shout out to @LLFourn for these suggestions.
* Improve/fix `LocalChain` documentation
* Refactor `TxGraph::missing_blocks` to make it more explicit that
`last_block` has state.
* `update_local_chain` method of `EsploraExt` and `EsploraAsyncExt` now
returns a `local_chain::Update` instead of just a `CheckPoint`.