db: Add index on utxoset.spendheight to speed up blockchain reorgs

Otherwise we would be doing a table scan per block being reorged/rescanned.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

wallet/db.c

@@ -227,6 +227,9 @@ char *dbmigrations[] = {
     " satoshis BIGINT,"
     " PRIMARY KEY(txid, outnum));",
     "CREATE INDEX short_channel_id ON utxoset (blockheight, txindex, outnum)",
+    /* Necessary index for long rollbacks of the blockchain, otherwise we're
+     * doing table scans for every block removed. */
+    "CREATE INDEX utxoset_spend ON utxoset (spendheight)",
     NULL,
 };