Support missing or empty X-B3-Sampled

[drolando]

Right now, we are ignoring the pyramid_zipkin path/route blacklist in firehose mode, which means firehose mode is emitting spans for things like /status.

We can support this more generally by not firehose tracing when X-B3-Sampled=0 and only sampling when it is 1 or absent (see https://github.com/openzipkin/b3-propagation for details on this)

This will be a little bit tricky because pyramid_zipkin and py_zipkin do not correctly implement this nuance of the b3 spec. py_zipkin's create_http_headers_for_new_span doesn't support X-B3-Sampled being absent, and pyramid_zipkin does not set zipkin_attrs.is_sampled to None when a trace decision is deferred (i.e. absent X-B3-Sampled)