Protect ubik cache accesses

Protect ubik cache accesses

Currently, ubik application cached data could be updated and read by
different threads simultaneously. Add a mechanism in ubik for
protecting accessing and updating the cached data. This adds the
function ubik_CheckCache to do this, and removes ubik_CacheUpdate as
an exported function (since it's not safe).

Update all callers to use the new mechanism. In ptserver, remove the
'initd' variable; just rely on cachedVersion and ubik_CheckCache to
tell us when to re-read the database. Remove db.lock in buserver and
cheader_lock in kaserver, which served similar (though not completely
threadsafe) protection as ubik_CheckCache. Add the ubik database lock
cache_lock to protect the application cache.

Change-Id: I857a67d410e2c539197c5212c3b114c3fd0403c2
Reviewed-on: http://gerrit.openafs.org/1546
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Russ Allbery <rra@stanford.edu>