LUClusterVerifyConfig: Share BGL, acquire all locks in shared mode

Instead of acquiring the BGL in exclusive mode (which blocks all other
operations), we acquire all locks for groups, nodes and instances in
shared mode before verifying the configuration.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>