Problem: zyre_node's own_groups attr is used as list but is hash
Problem: zyre_node's own_groups attr is used as list but is hash

Solution: The hash of zyre node's own_groups contains a reference to a
zyre_group which is never referenced and which is not valid anyway. Thus
replace the hash with a plain list which is sufficient for tracking the
groups we are in.

twhittock/zyre,twhittock/zyre,zeromq/zyre,zeromq/zyre,twhittock/zyre,twhittock/zyre,twhittock/zyre,twhittock/zyre,twhittock/zyre,zeromq/zyre,zeromq/zyre,zeromq/zyre,zeromq/zyre,zeromq/zyre,twhittock/zyre,zeromq/zyre