Restored glue/LDBDWClient.py, for ligolw_segments_from_cats_dqsegdb

In running the DQSegDB test suite (/home/detchar/dqsegdb/test_segments_machine.sh on the CIT cluster) on the modified files in MR 84, the ligolw_segments_from_cats_dqsegdb client tool was found to be broken, since glue/LDBDWClient.py was missing, having been deleted in MR 74.

This MR restores glue/LDBDWClient.py (having done so with git checkout 7c93764~1 -- glue/LDBDWClient.py, where the commit 7c937646 was found with git log -- glue/LDBDWClient.py).

(Subsequent tests, with glue/LDBDWClient.py having been manually added to the site packages dir (/root/venv_p3_glue_from_repo/lib64/python3.6/site-packages/glue/), were successful - e.g., /home/robert.bruntz/git_work/dqsegdb_2021.11.30/dqsegdb/bin/ligolw_segments_from_cats_dqsegdb --separate-categories --individual-results --segment-url https://segments-backup.ligo.org --veto-file /home/detchar/dqsegdb/H1L1V1-S6_CBC_HIGHMASS_D_OFFLINE-961545543-0.xml -s 931035615 -e 940000000 -o /tmp/cats_short_test_output).