Refs #76: use act_proto_ver to control format of STR
Refs #76: renaming. when we refers to group_protocol_version
1. core->proto_ver and core->group->quorum.gcs_proto_ver mean highest common supported version
2. core->group->gcs_proto_ver means its own max supported version. although name is bit confusing.
Refs #76: use repl_proto_ver instead of gcs_proto_ver to control STR message format
Refs #76: allow gcs protocol version to downgrade
Merge codership/gh76 upto 7ceda5e834471d44242e13961baf1b9660d81c4b