wallet: Add read-only flag to extracted queries

This gets rid of the two parallel execution paths of read-only and write
queries, by explicitly stating with each query whether it is a read-only
query, we only need to remember the ones marked as write queries.

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

devtools/sql-rewrite.py

@@ -28,7 +28,8 @@ struct db_query db_${f}_queries[] = {
     {
          .name = "${elem['name']}",
          .query = "${elem['query']}",
-         .placeholders = ${elem['placeholders']}
+         .placeholders = ${elem['placeholders']},
+         .readonly = ${elem['readonly']},
     },
 % endfor
 };