[keyserver] Fix stalling of keyserver public key script

Summary: Should've noticed this in D8448. We initialize the MySQL connection but never close it, which is why the script stalls. This is addressed by using the main util (all scripts are supposed to use this).

Test Plan: Run script locally, confirm it no longer stalls

Reviewers: varun

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8449