"ChangeSet" API is error prone in multi-threaded contexts

[LLFourn]

"ChangeSet" API is error prone in multi-threaded contexts

[evanlinjin]

Another option is to:

1. Use sparse_chain::ChangeSets with ChainGraph (instead of another ChangeSet type for chain_graph).
2. Add ability to add auxiliary transactions directly into TxGraph of ChainGraph.

The API will be used like this:

1. ChainGraph::apply_changeset() will error if changeset has missing full transactions.
2. We can poll request for the missing txs from electrum server.
3. Add the missing transactions into ChainGraph without ChainPosition.
4. ChainGraph::apply_changest() again.