set journal mode to wal when creating db.

Trying to work around sqlite3.OperationalError: disk I/O error
errors in CI that I can't reproduce locally.

Also this allows reads to happen while writes are occurring.
This should permit using the web interface for browsing while
a large write load (e.g. bulk data loading from another tracker)
is happening. Right now autogenerating issues locks up the tracker
for reading with an error that the database is locked.

Author: rouilj

roundup/backends/back_sqlite.py

@@ -203,6 +203,8 @@ def open_connection(self):
             self.sql('create index ids_name_idx on ids(name)')
             self.create_version_2_tables()
             self._add_fts5_table()
+            # Set journal mode to WAL.
+            self.conn.execute('pragma journal_mode=wal')
 
     def create_version_2_tables(self):
         self.sql('create table otks (otk_key varchar, '