Track and report task result serialisation time.

 - DirectTaskResult now has a ByteBuffer valueBytes instead of a T value.
 - DirectTaskResult now has a member function T value() that deserialises valueBytes.
 - Executor serialises value into a ByteBuffer and passes it to DTR's ctor.
 - Executor tracks the time taken to do so and puts it in a new field in TaskMetrics.
 - StagePage now reports serialisation time from TaskMetrics along with the other things it reported.