feat(chain)!: wrap `TxGraph` txs with `Arc`

feat(chain)!: wrap `TxGraph` txs with `Arc`

Wrapping transactions as `Arc<Transaction>` allows us to share
transactions cheaply between the chain-source and receiving structures.
Therefore the chain-source can keep already-fetched transactions (save
bandwidth) and have a shared pointer to the transactions (save memory).

This is better than the current way we do things, which is to refer back
to the receiving structures mid-sync.

Documentation for `TxGraph` is also updated.