Additional functionality for rxperf

Additional functionality for rxperf

Add the ability to initiate multiple pthread threads issuing
calls on the same connection to the server.

Add ability to set max window size.

Add ability to adjust min peer timeout.

Change-Id: I896650b033ae0adfa42d3e5831ff799ca1331bd5
Reviewed-on: http://gerrit.openafs.org/2835
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>