libafs: add replicated connection pool

libafs: add replicated connection pool

keep pool of connections to use for replicated volumes,
so we can have a separate idle time setting

(cherry picked from commit cd1f72649650404581cfcdcf3beeeaf2bb960bd6)
Reviewed-on: http://gerrit.openafs.org/6546
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Change-Id: I056ba28d11313c9925df63869e0c55a1a4f132da
Reviewed-on: http://gerrit.openafs.org/6610
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>