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: I5604fa3cc8a334606f5e3535aed6f6cedcb5f167
Reviewed-on: http://gerrit.openafs.org/2103
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>