Deprecations

[dd-trace-api] Deprecate scope finishOnClose and continuation close #1424
- Close spans manually instead of relying on finishOnClose
- Use Continuation.cancel() instead of Continuation.close()

Improvements

[Performance] Avoid allocation of byte[] in MessagePacker.writeString #1435
[Performance] Intern UTF8 encoding of well known constants to reduce allocation#1430
[Performance] Replace collection.size() > 0 with not collection.isEmpty() #1420
[Performance] Avoid allocating an array when getting first header value #1418
Add instrumentation for OkHttp 2.2+ #1402 (Thanks @jerchung for the contribution)
Reduce finalize() logging in tracer #1416
Change logging for ScopeEventFactory creation exception #1417

OpenTracing Split #1354, #1439, #1429
- Note: Some unchecked casting will no longer be possible
  - Spans passed into TraceInterceptor will no longer be castable to io.opentracing.Span or datadog.opentracing.DDSpan
  - Spans returned from OpenTracing interfaces will no longer be castable to datadog.opentracing.DDSpan
Do not stop profiling on fatal exceptions #1421
Upgrade jmxfetch to 0.36.1 and integrations-core modules to 7.19.0 #1410
Upgrade Byte Buddy to 1.10.10 #1409