Fix wallet sync not finding coins of addresses which are not cached

Fix wallet sync not finding coins of addresses which are not cached

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).