Previously, electrum-based blockchain implementations only synced for
`scriptPubKey`s that are already cached in `Database`.
This PR introduces a feedback mechanism, that uses `stop_gap` and the
difference between "current index" and "last active index" to determine
whether we need to cache more `scriptPubKeys`.
The `WalletSync::wallet_setup` trait now may return an
`Error::MissingCachedScripts` error which contains the number of extra
`scriptPubKey`s to cache, in order to satisfy `stop_gap` for the next call.
`Wallet::sync` now calls `WalletSync` in a loop, cacheing inbetween
subsequent calls (if needed).
This is a continuation of the #651 fix. We should also check whether the
same bug affects esplora as noted by @afilini. To achieve this, I've
introduced a `ConfigurableBlockchainTester` trait that can test multiple
blockchain implementations.
* Introduce `ConfigurableBlockchainTester` trait.
* Use the aforementioned trait to also test esplora.
* Change the electrum test to also use the new trait.
* Fix some complaints by clippy in ureq.rs file (why is CI not seeing
this?).
* Refactor some code.
