Revert "Stop acquiring BGL for LUXI queries"

This reverts commit 0fa753ba.

Turns out there are more queries acquiring locks than we'd like. This
patch goes to version 2.6 and a separate patch fixes the immediate
issues in LUClusterVerifyConfig.

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