vlserver: Access cache via vl_ctx

The vlserver application-level ubik cache (which consists of
HostAddress, ex_addr, and cheader) is currently being accessed via
global variables everywhere. Instead, access these via the new vl_ctx
struct that is passed to functions during a transaction, so we have
the ability to modify the cache without making all changes visible as
we change it.

Reviewed-on: http://gerrit.openafs.org/2105
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 1f532d099b8b084d43dd0140890448464325b602)

Change-Id: I161b6c41e104af045941663f78148037313036ee
Reviewed-on: http://gerrit.openafs.org/9422
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>