Support CompositeTracers within the Java SDK

As covered in the RFC for RTO - https://github.com/couchbaselabs/sdk-rfcs/blob/master/rfc/0035-rto.md#composite-tracer:

Future Enhancements
Composite Tracer

The CompositeTracer is a vasard over a collection of sub-Tracer implementations and as a span finishes it passes the span onto each inner_ Tracer_. This would be useful to combine the functionality of a number of other Tracer implementations.

NOTE: This was moved to be a future enhancement because typically tracer implementations require custom Span implementations as they (likely) include additional information. eg the ThresholdLoggingTracer described above includes a summary object. This means the generic interface ISpan is not as useful.

This is a JCBC raised to track the addition of CompositeTracers within the Java SDK (have not created one for each SDK individually at this point) as it has been specifically requested.