ubik: Introduce version lock

ubik: Introduce version lock

The "version" lock is a new lock that protects the database version
information.  The goal is to allow the beacon thread to use the
protected values without blocking for an extended period of time,
which could occur if it was using the database lock.

Reading requires holding either lock, while writing requires holding
both locks.

The following values are protected:
	ubik_epochTime
	db->version
	db->flags
	db->tidCounter
	db->writeTidCounter

Based on analysis and design work from Jeffrey Hutzelman.

Change-Id: Ib6e67360807eed8c36e35ec27d1eb938ac899e22
Reviewed-on: http://gerrit.openafs.org/4158
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>