[bdk_chain_redesign] Various tweaks and fixes

[bdk_chain_redesign] Various tweaks and fixes

The `ConfirmationHeight` trait has been removed in favour of a second
method on the `Anchor` trait: `confirmation_height_upper_bound()`.

Methods `try_balance_at()` and `balance_at()` of `IndexedTxGraph` have
been removed as they do not provide additional functionality.

`IndexedTxGraph::insert_relevant_txs` now uses two loops, the first loop
indexes all transactions first. This is done as some indexes require
ancestor transactions to be indexed first and we cannot guarantee that
the input transactions are in topological order.