Request : fs allow user allocate buffer, allow write with multiple buffers at a time, remove unique_ptr enforcement

[Diarica]

Currently file_req::read is allocate buffer memory with std::unique_ptr, hopefully allow user allocate it.

[Diarica]

Also in fs write_sync, could we provide a blocking write function and without unique_ptr enforcement?

[Diarica]

And please, support write file with multiple buffers. Currently no matter write or write_sync, there are only support 1 buffer at a time.