Follow-up adjustments to the recent performance improvement changes

[knyar]

• Rename vendored resty_counter library to make it easier to use without Resty.
• Make tests actually use resty_counter.
• Make inc() on a gauge skip counters to maintain strong consistency.
• Move sync_interval to init_worker.
• Add more details to README and code comments.

@fffonion, since this involves your improvements, may I ask you to take a quick look at this please before I merge it? Please let me know if you would prefer to be mentioned in the README.md in a different way, or not mentioned at all. Thanks!

[coveralls]

Coverage decreased (-0.5%) to 93.313% when pulling d8c2813d3a94edae6d18d90475e49f719682cc52 on dev into c20e7df2e7d8296350925dddcacb20c5813d0327 on dev_tmp.