msg/async/ProtocolV2: refuse incoming reconnect attempt intended for someone else

msg/async/ProtocolV2: refuse incoming reconnect attempt intended for someone else

The same scenario that was possible on client_ident is also possible
for reconnect: on the accepting side we recognized the client_addrs
but the client is trying to reach someone else.  Since the cookie is a
random 64-bit value it is unlikely, but still possible, that we will
match an existing session that is not for the client.

Signed-off-by: Sage Weil <sage@redhat.com>