* trace sdk initial commit.
* fix imports and comments.
* remove tracestate
* split trace.go
* add attribute over limit test.
* add comments and restructure span.go and tracer.go
* refactor MessageEvent
* defer unlock
* some more cleanup in span.go
* rename *MessageEvent* to *Event*
* cleanup comments in trace_test.go
* fix typos.
* return full string ID for traceID and spanID.
