Consider using async channels instead of `crossbeam`

[msdrigg]

I was following the issue here eaze/tracing-honeycomb#9, and it seems that some environments are having issues with this library (I myself am not one of these currently). There is a fork of this library that uses async channels (https://github.com/agrinman/libhoney-rust) and this has resolved those issues for some.

Additionally, it just seems like the more cohesive approach would be to use async channels because this library is built on async anyway.

[nlopes]

Indeed it is. I'll take a look at this - has been a while. I think when I first took a stab at the lib async wasn't stable.