Log membership changes in GossipRouter

GossipRouter logging is all or nothing, or it logs every message received or it logs nothing.
In production, it may be useful to log only the membership changes, messages like add <member> to group <group>.

My idea is as follow: adding another option for -dump_msgs; So, instead of being a boolean, something like

-dump_msgs NONE|REGISTRATION|ALL

-dump_msgs false will be translated to NONE and -dump_msgs true to ALL