unix cm rx-oblivious connection pooling

unix cm  rx-oblivious connection pooling

Implements a connection pool for Rx client connections with the
same credentials.  The code trivially avoids the limit on Rx
call channels, without touching Rx directly.  The conn call limit
is known to be hit in cache-bypass, and probably other use cases,
so there is an incentive to address it potentially sooner than
larger Rx changes are ready to merge.

Upgrade to exclusive lock before calling find_preferred_connection.
Unset trace option.  Fix a warning around modular increment of
select_index, we'll go with the change suggested by Marc, I don't
see a real need to save the value mod CVEC_LEN.

Change-Id: I956aa22cd52b1c43187c7e03230e36820ba11c4d
Change-Id: Ie5f936ba912bcfe0544dbc1d035bb74e5ddc50f8
Reviewed-on: http://gerrit.openafs.org/2216
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>