Perf: limit degree of concurrency on ZipkinHttpReporter

[Aaronontheweb]

Kind of need #4 to do this properly. Some of our applications using Petabridge.Tracing.Zipkin are capable of toppling over a smaller Zipkin instance with little effort (although to be fair, the environment where this has occurred is Docker for Windows which isn't particularly robust.)

However, this will be an issue in sufficiently busy systems. I think we should consider using some type of exponential back-off-type mechanism or rate-limiting to help protect Zipkin endpoints from being overwhelmed by busy transports.

Honestly though, in a busy enough system HTTP ain't going to cut it. Might be better off just implementing gRPC #17 or Azure Event Hubs #18