QPID-7576: [Java Broker] refactoring related to message metadata

This addresses review comments from alex:
 * remove return value from ConsumerTarget#send
 * call StoredMessage.getContentSize() instead of StoredMessage.getMetaData().getContentSize()
 * assume _handle in AbstractServerMessageImpl is not null
note that the last point differs from review suggestion in that _handle is not explicitly guarded against null.
13 files changed