ubik: add interface for reading during write locks

ubik: add interface for reading during write locks

Add ubik_BeginTransReadAnyWrite, which allows for reading from the
database, even while there is a conflicting ubik write lock. Reads are
still blocked while the local database is updating due to a write
transaction commit.

Reviewed-on: http://gerrit.openafs.org/2592
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit fd7f7469bdbde24320c3ac8b4f7015d495b47bff)

Change-Id: I46cf35beeea6dc997976ea2c33e45c1a5f4904df
Reviewed-on: http://gerrit.openafs.org/6261
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>