refactor(aio): simplify disk_engine interfaces

[neverchanje]

class disk_engine : public utils::singleton<disk_engine>
{
public:
    // asynchronous file read/write
    disk_file *open(const char *file_name, int flag, int pmode);
    error_code close(disk_file *fh);
    error_code flush(disk_file *fh);
    void read(aio_task *aio);

disk_engine was somehow designed as a middle layer between "file_io" API and "aio_provider" API.

For example, file::flush forwards the call to disk_engine::flush, then forwards to aio_provider::flush.

/*extern*/ error_code flush(disk_file *file) { return disk_engine::instance().flush(file); }

error_code disk_engine::flush(disk_file *fh)
{
    if (nullptr != fh) {
        auto df = (disk_file *)fh;
        return _provider->flush(df->native_handle());
    } else {
        return ERR_INVALID_HANDLE;
    }
}

As a middle layer, disk_engine adds complexity of the code but doesn't bring abstraction above aio_provider. So I plan to reduce the functionalities of this class. I may eventually remove this class.