SqliteVtabSyncCallback
This method signals the start of a two-phase commit on a virtual table. This method is optional. The xSync pointer of sqlite3_module may be NULL.
This method is only invoked after a call to the xBegin method and prior to an xCommit or xRollback. In order to implement two-phase commit, the xSync method on all virtual tables is invoked prior to invoking the xCommit method on any virtual table. If any of the xSync methods fail, the entire transaction is rolled back.