- update all fields `immature`, ` trusted_pending`, `unstrusted_pending`
and `confirmed` to use the `bitcoin::Amount` instead of `u64`
- update all `impl Balance` methods to use `bitcoin::Amount`
- update all tests that relies on `keychain::Balance`
Coinbase transactions cannot exist in the mempool and be unconfirmed.
`TxGraph::try_get_chain_position` should always return `None` for coinbase
transactions not anchored in best chain.
