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).

src/error.rs

@@ -13,7 +13,7 @@ use std::fmt;
 
 use crate::bitcoin::Network;
 use crate::{descriptor, wallet, wallet::address_validator};
-use bitcoin::OutPoint;
+use bitcoin::{OutPoint, Txid};
 
 /// Errors that can be thrown by the [`Wallet`](crate::wallet::Wallet)
 #[derive(Debug)]
@@ -125,6 +125,10 @@ pub enum Error {
     //DifferentDescriptorStructure,
     //Uncapable(crate::blockchain::Capability),
     //MissingCachedAddresses,
+    /// [`crate::blockchain::WalletSync`] sync attempt failed due to missing scripts in cache which
+    /// are needed to satisfy `stop_gap`.
+    MissingCachedScripts(MissingCachedScripts),
+
     #[cfg(feature = "electrum")]
     /// Electrum client error
     Electrum(electrum_client::Error),
@@ -145,6 +149,16 @@ pub enum Error {
     Rusqlite(rusqlite::Error),
 }
 
+/// Represents the last failed [`crate::blockchain::WalletSync`] sync attempt in which we were short
+/// on cached `scriptPubKey`s.
+#[derive(Debug)]
+pub struct MissingCachedScripts {
+    /// Number of scripts in which txs were requested during last request.
+    pub last_count: usize,
+    /// Minimum number of scripts to cache more of in order to satisfy `stop_gap`.
+    pub missing_count: usize,
+}
+
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{:?}", self)