wallet: Write bestblock to watchonly and solvable wallets

When migrating, we should also be writing the bestblock record to the
watchonly and solvable wallets to avoid rescanning on the reload as that
can be slow.

src/wallet/wallet.cpp

@@ -3927,6 +3927,13 @@ bool CWallet::ApplyMigrationData(MigrationData& data, bilingual_str& error)
         }
     }
 
+    // Get best block locator so that we can copy it to the watchonly and solvables
+    CBlockLocator best_block_locator;
+    if (!WalletBatch(GetDatabase()).ReadBestBlock(best_block_locator)) {
+        error = _("Error: Unable to read wallet's best block locator record");
+        return false;
+    }
+
     // Check if the transactions in the wallet are still ours. Either they belong here, or they belong in the watchonly wallet.
     // We need to go through these in the tx insertion order so that lookups to spends works.
     std::vector<uint256> txids_to_delete;
@@ -3937,6 +3944,18 @@ bool CWallet::ApplyMigrationData(MigrationData& data, bilingual_str& error)
         LOCK(data.watchonly_wallet->cs_wallet);
         data.watchonly_wallet->nOrderPosNext = nOrderPosNext;
         watchonly_batch->WriteOrderPosNext(data.watchonly_wallet->nOrderPosNext);
+        // Write the best block locator to avoid rescanning on reload
+        if (!watchonly_batch->WriteBestBlock(best_block_locator)) {
+            error = _("Error: Unable to write watchonly wallet best block locator record");
+            return false;
+        }
+    }
+    if (data.solvable_wallet) {
+        // Write the best block locator to avoid rescanning on reload
+        if (!WalletBatch(data.solvable_wallet->GetDatabase()).WriteBestBlock(best_block_locator)) {
+            error = _("Error: Unable to write solvable wallet best block locator record");
+            return false;
+        }
     }
     for (const auto& [_pos, wtx] : wtxOrdered) {
         if (!IsMine(*wtx->tx) && !IsFromMe(*wtx->tx)) {