rxperf: use parallel connections

rxperf: use parallel connections

When the number of threads exceeds RX_MAXCALLS allocate additional
rx connection objects.

Reviewed-on: http://gerrit.openafs.org/3285
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit df682aa34170d285247c5486050db3ee7d3774f0)

Change-Id: Ia699fe0230e6dde2bc1f270d766f512f2693c94f
Reviewed-on: http://gerrit.openafs.org/8421
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>