Job execution buffer has proven unnecessary, and just wastes time

Remove event timing that doesn't work

Fixes #29

This is a bit vestigial. At some point, I thought there'd be more to do between queueing jobs and running them.

Fixes #43

Set the `DOING_CRON` constant as early as possible

In particular, the `DOING_CRON` constant should be set before any client code is loaded.

Fixes #44

Introduce CLI commands to view plugin data

See #32

Just needs the event data itself to be filled in

Add a description for the one-time fixers set of commands

Introduce command to flush plugin's cache

See #28

Prompt between calculating how many entries there are to remove and performing any further queries.

Introduce CLI command to purge events

Improve messages, making them translatable. Also, simplifies how $dry_run is checked, reducing duplication.

Includes basics of a purge command

Purge completed events more frequently

Fixes #13

Purge completed events more-frequently

Fixes #13 

Purge completed events more-frequently, in case site has sufficient volume that there are large quantities to remove

Fixes #13