Most of them are trivial, except for the thread_sync_t refactoring.
We must not invoke memset() on non-POD objects.

mtflush_work_initialized: Remove. Refer to mtflush_ctx != NULL instead.

thread_sync_t::thread_sync_t(): Refactored from
buf_mtflu_handler_init().

thread_sync_t::~thread_sync_t(): Refactored from
buf_mtflu_io_thread_exit().