vlserver: Add a struct for trans-specific data

Instead of passing a ubik_trans pointer to many functions inside the
vlserver, pass a vlserver-defined vl_ctx struct, so we can add new
things to keep track of in a transaction that are not part of ubik.

Reviewed-on: http://gerrit.openafs.org/2104
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 3bf0350218dcf4d6e086941fac394769b6b9cc4a)

Change-Id: Ic224f1fab7505b486baa0a41e5481bc615f00826
Reviewed-on: http://gerrit.openafs.org/9421
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>