Kamailio 4.2 introduces the option to do the accounting to database using the asynchronous framework from core, provided that database connector module has support for it. This feature can be enabled by setting the acc module parameter db_insert_mode to 2.
async_workers=4 ... modparam("acc", "db_insert_mode", 1)
At this moment, db_mysql module is able to do asynchronous queries, so it has to be used for accounting. Expect other database drivers to implement this feature in the near future. If db_insert_mode is 2 and database connector module doesn't support asynchronous query, then normal insert is performed.