Allow to share multicast socket within jvm

Due to bugs in jvm (specifically OSX), running zen discovery tests causes for "socket close" failure on receive on multicast socket, and under some jvm versions, even crashes. This happens because of the creation of multiple multicast sockets within the same VM. In practice, in our tests, we use the same settings, so we can share the same multicast socket across multiple channels.
This change creates an abstraction called MulticastChannel, that can be shared, with ref counting. Today, the shared option is only enabled under OSX.