Make write buffers bigger, but flush early

Make write buffers bigger, but flush early

This patch
 - makes the server write buffer bigger: 64k,
 - makes the regular write buffer bigger: 32k,
 - tries to flush the write buffer starting at 4K.

Before this patch, a client got disconnected if the buffer flushing at 4k
failed, now regular clients can store up to 32k and servers up 64k even
if flushing is not possible at the moment (e.g. on slow links).