vlserver: --enable-ubik-read-while-write configure option

vlserver: --enable-ubik-read-while-write configure option

Commit a0f416e3504929b304fefb5ca65e2d6a254ade2e unconditionally turned
on the new ubik_BeginTransReadAnyWrite functionality for the vlserver,
which allows us to read data from ubik during a conflicting ubik write
lock.

This feature is not ready for production use. Make it a build time
option, marked as experimental, until more testing can be done.

Change-Id: If64702e7a7ed2340066df5faf82ce8b0875fc610
Reviewed-on: https://gerrit.openafs.org/12240
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>