SpanKind support in opencensus-java

[HailongWen]

census-instrumentation/opencensus-proto#51 added SpanKind. We should add the initial support.

Another thread is discussing whether to add consumer/producer in the type census-instrumentation/opencensus-specs#54. This could be merged after the decision is made.

[bogdandrutu]

I think we can add the SpanKind right now and extend later if needed, I would suggest that we update the docs related to HTTP/gRPC span names to not include "Sent." and "Recv." which we will be able to determine from the SpanKind.

Also we need to update the https://github.com/census-instrumentation/opencensus-java/blob/master/impl_core/src/main/java/io/opencensus/implcore/trace/export/InProcessSampledSpanStoreImpl.java to map not only by name but also by SpanKind.

[bogdandrutu]

This is also related #1226

[voidzcy]

gRPC is trying to migrate to use SpanKind (see grpc/grpc-java#6732), but since Span name is changed, this seems to be a "flip the switch" change for traffic monitoring pipelines. How should we migrate this in gRPC for OSS without breaking open-source users' traffic monitoring pipelines?