Headers: data in some headers might be too large in big clusters

Similar to JGRP-1710 and JGRP-1713, investigate headers which might become too big, and move their contents into the message body. Check all headers which contain

• a view
• a digest
• a list of members

Candidates:

• PingHeader.

Probable solution: move PingHeader.PingData from the header to the message body