Eliminate zeroing of buffers while writing the ASYNCIO journal

By default ASYNCIO using TimedBuffer are zeroing the ByteBuffers used to perform the write on the journal: under load TimedBuffer can be big enough that zeroing would cost the same as copying the actual content into it, hence saving the zeroing is quite beneficial to save CPU time and increase IOPS.

NIO and MAPPED doesn't need it because they already use directly the TimedBuffer's accumulation buffer to perform writes (that's never zeroed, because it already contains the content that they expect to write into the journal).