NEXUS-5790: Gave up NIO

As it was producing slightly worse results than
old IO. Reason is probably that no native buffer helps here,
as all response has is a ServletOutputStream, with no way
(I found using google) to get directly the socket channel
from Jetty, hence only Channels.newChannel wrapper was
possible around servlet response stream, that is
already a wrapper around Jetty Buffer that is a wrapper
around... etc.

I bet Jetty folks did optimize the best how ServletOutputStream
is handled on their side, and along with buffer size of
16KB that got proven as "best choice", Nexus now produces
download speeds of around 400 MB/s instead of 100 MB/s as
before.