Improve performance of all_tag_counts on large tables.

Refactored the sub select to actually do a separate query. This improves the performance of the count methods drastically for MySQL/SQLite. I was unable to test this under the pg driver, however that should work fine.

All tests are passing on my side, and I had a 100+ second query drop to 2 seconds on my production server. Let me know if there is anything else you need.

If there are any regressions that you see, let me know and I can refactor this until it is performant and works perfectly for everyone.

Thanks again for a great lib.