viced: disable hot threads

Turn off the rx hot threads feature in the file server.  This feature
was an old optimization intended to reduce context switching, however
generally makes performance worse on modern hardware.

Performance improvements from disabling hot threads was identified by
Simon Wilkinson (YFS) at the European AFS and Kerberos Conference (EAKC)
2014 at CERN.

Change-Id: Id3053a61ebdb2d49d2bf36ebe07a35cc07b5d65c
Reviewed-on: http://gerrit.openafs.org/10957
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>