Implement simple hash based sampler (sample request based on hash value of trace_id)

[jettify]

1. Create abstract base class SamplerAbc
2. Make current class subclass of SamplerAbc
3. Implement one more sampler class base and hash idea from brave

aiozipkin/aiozipkin/sampler.py

Lines 4 to 15 in 1879edd

	class Sampler:
	def __init__(self, *, sample_rate=1.0, seed=None):
	self._sample_rate = sample_rate
	self._rng = Random(seed)
	def is_sampled(self, trace_id):
	if self._sample_rate == 0.0:
	sampled = False
	else:
	sampled = self._rng.random() <= self._sample_rate
	return sampled

But sampling logic ported from here:
https://github.com/openzipkin/brave/blob/967e48507f6267c07e3238b8f30575c683690ef6/brave/src/main/java/brave/sampler/BoundarySampler.java#L43-L47