streamchain

There are presently two etcdraft protos around metadata.  One is the
metadata stored in the config and it is named 'Metadata', the other is
the metadata stored in each block, this is named 'RaftMetadata'.  This
causes confusion when reading the code.  This CR transforms those names
to be:

 Metadata -> ConfigMetadata
 RaftMetadata -> BlockMetadata

Change-Id: Ia0394ebe78f5541996c010c3c67d760f336f75d8
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>