Add support for b3multi

[zdmytriv]

Is your feature request related to a problem?
Need support for b3-multi for nginx module. https://github.com/openzipkin/b3-propagation#multiple-headers

Describe the solution you'd like
Implement b3-multi in similar fashion like it is with simple b3 just do not concat trace-span-sampled into on header.

Right now header looks like

"B3": "2605e09e4efca19056beee12767f6287-426bf077362e8477-1",

for b3-multi there should be 3 headers

X-B3-TraceId: 2605e09e4efca19056beee12767f6287
X-B3-SpanId: 426bf077362e8477
X-B3-Sampled: 1

[kuzaxak]

@ThomsonTan Could you please have a look?

[seemk]

Fixed with #495