Catch callback exceptions to avoid deadlock

If an event callback fatals or times out, it can deadlock. By catching the fatal (PHP 7+), deadlock is avoided.

Fixes #105
Fixes #117

Ensure WP-CLI's `--quiet` flag is respected

From `line()`'s inline doc:

> Display informational message without prefix, and ignore `--quiet`.

Removing unused global $wpdb variable

The `$wpdb` variable is not being used in \Automattic\WP\Cron_Control\CLI\Events::get_events() methods and thus is not necessary to be made accessible in there.

Remove the plugin's table when a subsite is deleted from a multisite

Add example for concurrency

Fixes #107

Allow whitelisted events to run concurrently

Most events, due to alloptions inconsistencies, cannot run in parallel. Certain events, however, may be acceptable.

Fixes #106.

For monitoring and alerting, include total of pending events in list endpoint response

A dedicated endpoint is overkill, as the list endpoint is central to the plugin operating as expected.

Fixes #100

Ensure `DOING_CRON` is set for all REST requests

If code is loaded conditionally when `DOING_CRON` is set, not setting it for all of the plugin's REST endpoints can lead to unexpected behaviour.

For example, duplicate events can be created infinitely, in combination with the skip-empty-event functionality.

Fixes #97

Clear Event Store caches when table is created

Convert to custom table