[keyserver] Rename COMM_MYSQL env variables to COMM_DATABASE

Summary: See feedback here.

Test Plan: I git grep for each variable name to make sure the spelling was correct, and I made sure the number of results matched my expectations

Reviewers: jonringer-comm, atul, palys-swm

Reviewed By: jonringer-comm

Subscribers: Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4515