LUGetTags: Acquire locks in shared mode

Retrieving tags can be done while the lock is shared. Only writing
needs to be exclusive.

Also add a FIXME for cluster tags, where the code currently doesn't
use any locks except the config lock.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>