Windows: Add caching to cm_GetAddrsU

Cache the results of VL_GetAddrsU queries and reuse the results
for subsequent calls when possible.

Change-Id: I7e2b086ec311208a46439588bc820a1929d2b2b9
Reviewed-on: http://gerrit.openafs.org/10764
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>