[core] Add new DatadogSampler (#1020)

* Add py2/3 re.Pattern type to ddtrace.compat

* Add in token bucket rate limiter

* Add prototype of new DatadogSampler

* fix linting

* fix compat check for re.Pattern

* start work on tests

* fix up issues

* work on tests

* fix flake issues

* Add rate limiter tests

* return False on an error

* add more SamplingRule test cases

* Add RateLimiter __repr__ and __str__

* fix py.27 tests... float division...

* remove debug print

* update api, docs, todos, and tests

* fix setting span.sampled

* narrow lock range

* use monotonic time

* s/transcation/request/

* always apply a default

* add fallback to priority sampler

* add test cases for when priority sampling is enabled

* fix up for comments

* [core] Tag sampling decision (#1045)

* [core] Tag sampling decision

* fix Flask tests

* [core] Add rate limit effective sample rate

* configure RateLimiter.effective_rate

* Set metrics not tags

* average with last window

* fix context tests

* remove _dd.user_psr

* add tests and fix up

* remove unused constants

* Remove SamplingRule resource and tags