vlserver: Access cache via vl_ctx

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.

Change-Id: I24bd5495539057d169aef4b5b0b6850b77172cab
Reviewed-on: http://gerrit.openafs.org/2105
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>