Improve performance of Message#readHeader

A CPU hot spot highlighed by profiling via JFR:

Stack Trace	Sample Count	Percentage(%)
java.lang.reflect.Constructor.newInstance(Object[])	71	2.392
   java.lang.Class.newInstance()	71	2.392
      org.jgroups.Message.readHeader(DataInput)	71	2.392

I'd have expected the reflective constructor to perform well on a recent JVM, but apparently it's not in this case. A theory is that the Class type being unknown makes this code harder to optimise; needs to be looked into.

It might be possible to patch the ClassConfigurator to provide instances of the required Header type rather than returning the class, and optimise that instead.