The `WP_Query` approach is excessive

PHPUnit 6 drops the `PHPUnit_Framework_TestCase` class, breaking all tests derived from Core. This fix is derived from Core's solution; see https://core.trac.wordpress.org/ticket/39822.

Review feedback: check that version is lower, to prevent loops if DB version is greater than expected

Also, `wp_rand()` is unnecessary here, `mt_rand()` is sufficient.

Removes a bit of unnecessary code and cleans up one of the stranger vestigial bits of this plugin.

Restore Jetpack Sync for CPT

Exclusion is causing sites to continually be out of sync.

Plugin could be half-activated, due to secret's absence, but Events Store would still capture data. As the rest of the plugin, notably its CLI commands, aren't loaded, this could store data into an unreachable hole. Also, with the introduction of a custom table, the table shouldn't be created when the environment isn't ready.

With unique constraints, this will help with situations where events with specific timestamps are changed