Regenerate trace ID for a thread after each send (#11)

* Regenerate trace ID for a thread after each send

Since threads can be run again for new traces, we should regenerate
after each "send" to make sure that a traceID only stays valid from teh
first span until the send (which sends the parent as well), then we can
regen a trace ID and start again if the same thread starts a new trace
with a new parent span.

* cargo fmt