tagger: add two feature-flags that change seriesByTag behavior

[mchrome]

This PR intends to fix two unintended (i believe) behaviors and adds two feature flags which affect what metrics are returned after filtering with seriesByTag.

Changes that work without feature flags

1. From seriesByTag invalid regular expression parsing #244.
   seriesByTag("name=test","env!=~stage|env") now returns both metrics.
2. Given:
   request_success_total.counter;app=test;project=Test;environment=TEST;t=q request_success_total.counter;app=test;project=Test;environment=TEST;t=qac
   Target seriesByTag('t!=~qac') didn't return anything.
   Now it returns request_success_total.counter;app=test;project=Test;environment=TEST;t=q
   This only happened when a term with !=~ was the first tag after sorting terms by cost.

Changes with feature flags

1. When use-carbon-behaviour=true.

• Terms with = op and empty value (e.g. t=) match all metrics that don't have that tag.

2. When dont-match-missing-tags=true.

• Terms with != and !=~ operators only match metrics that have that tag.
  For example, without dont-match-missing-tags=true seriesByTag('t!=abc') would match metric.name;tag=val .