github_event table has no indexes

[DanCech]

The github_event table does not have any indexes, which means that DELETE operations are very slow when the table has more than a few thousand rows. This results in the githubstats-parser job timing out, and log entries like:

lvl=eror msg="failed to process event" logger=archive-downloader error="Error 1205: Lock wait timeout exceeded; try restarting transaction" skip=false

This can be remedied by setting id as the primary key for the table.