Formerly, `Span.getAccumulatedMillis()` worked, but could returns
imprecise measurements, particularly local spans. This changes the
internals of Span to keep track of a start tick. Using this, it can
return a more precise `Span.getAccumulatedMicros()`.
To ensure this precision isn't lost in serialization, this adds a
json field `durationMicros`, which is only set when the span is stopped.
This is set instead of start tick because `System.nanoTime()` is JVM
specific and so cannot be used across the network. `durationMicros` uses
null instead of zero comparisons because nano time can be negative.
Fixes#312
Fixed integration tests to use ZipkinRule
Introduce assertj assertions (#315)
* Introduce assertJ assertions in some tests
* Fix code formatting to be inline with Spring rules
6b5fa63553