PHPUnit doesn't load the plugin until after it's called `wp_install()`, which blocks normal table creation and disables the plugin.

If table creation fails during install, or plugin is added to an installed site, create table in limited circumstances

Race conditions are a concern, so it'd be better not to create the table without some control over when it happens.

When events are capped in this way, if they don't appear in the reconstructed array, they can be continually rescheduled.

With the introduction of the block on existing-event creation introduced in 61597bc5, this approach should longer be necessary. The creation of duplicates is also greatly reduced by the table's unique key.

Fixes #94

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.