Allow samplers to modify tracestate (open-telemetry#988)

Fixes open-telemetry#856

## Changes
Added `Tracestate` to `SamplingResult`

Related [oteps](https://github.com/open-telemetry/oteps) open-telemetry/oteps#135

CHANGELOG.md

@@ -29,6 +29,8 @@ New:
   * `exception.escaped` semantic span event attribute was added
     ([#784](https://github.com/open-telemetry/opentelemetry-specification/pull/784),
     [#946](https://github.com/open-telemetry/opentelemetry-specification/pull/946))
+- Allow samplers to modify tracestate
+  ([#856](https://github.com/open-telemetry/opentelemetry-specification/pull/988/))
 - Update the header name for otel baggage, and version date
   ([#981](https://github.com/open-telemetry/opentelemetry-specification/pull/981))
 - Define PropagationOnly Span to simplify active Span logic in Context

specification/trace/sdk.md

@@ -99,6 +99,11 @@ It produces an output called `SamplingResult` which contains:
   * `RECORD_AND_SAMPLE` - `IsRecording() == true` AND `Sampled` flag` MUST be set.
 * A set of span Attributes that will also be added to the `Span`. The returned
 object must be immutable (multiple calls may return different immutable objects).
+* A `Tracestate` that will be associated with the `Span` through the new
+  `SpanContext`.
+  Note: If the sampler returns an empty `Tracestate` here, the `Tracestate` will be cleared,
+  so samplers should normally return the passed-in `Tracestate` if they do not intend
+  to change it.
 
 #### GetDescription