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.

Change-Id: I025e595ad699d5a969a0676691530d90c65f1920
Reviewed-on: http://gerrit.openafs.org/2592
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>