Improve plugin performance

[ilicfilip]

We got a report from the user that the plugin slowed down his site, by investigating I found that we do get_option( 'option_optimizer' ) on every page load in register_hooks method. The reason was to set the accessed_options array.

Fetching options shouldn't be necessary on every page load, especially not that early, so I have replaced initial value of accessed_options with empty array. Then on shutdown hook, after the page was loaded for the user, option_optimizer entry is updated with the collected data.

This should speed up the page loading for the user as is, but there are still some room for improvement:

1. We can add an option to disable / enable option tracking, that way user can keep the plugin activated but tracking (which can be performance costly) doesnt run all the time.
2. Also we can add a filter so active tracking can be fine tuned, for example tracking is active only for specific user role.

Both align with Settings page which is added in #59 (comment)

3. One more thing that we could do is that on every page load we store accessed options in temporary wp_option entry. Then after some time interval (every hour?) we update option_optimizer (which can be much larger) with collected data. The downside is that the stats would be updated with a delay, but if plugin is collecting data 24/7 then it maybe is not that important.

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[ilicfilip]

I have implemented 3. - the update interval is 5 minutes (but it is filterable).