[FAB-5534]: Add messages to request pvt data

This commit adds new message types to support of state transfer response
and request in context of private data. There is a need for two message
types: 1) given txids request private data; 2) include a response with
private data requested.

Change-Id: I417fc68706b7266797da221f95d2ea1949d3a741
Signed-off-by: Artem Barger <bartem@il.ibm.com>

Author: C0rWin