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.

These methods allow us to query for checkpoints contained within the
linked list by height. This is useful to determine checkpoints to fetch
for chain sources without having to refer back to the `LocalChain`.

Currently this is not implemented efficiently, but in the future, we
will change `CheckPoint` to use a skip list structure.

We impl `PartialEq` on `CheckPoint` instead of directly on `LocalChain`.
We also made the implementation more efficient.

This creates a checkpoint linked list which contains all blocks.

Previously, we would update the `TxGraph` and `KeychainTxOutIndex`
first, then create a second update for `LocalChain`. This required
locking the receiving structures 3 times (instead of twice, which
is optimal).

This PR eliminates this requirement by making use of the new `query`
method of `CheckPoint`.

Examples are also updated to use the new API.

These methods are no longer needed as we can determine missing heights
directly from the `CheckPoint` tip.

This gets the genesis hash of the env blockchain.

We ensure that calling `finalize_chain_update` does not result in a
chain which removed previous heights and all anchor heights are
included.

…equests