Frequently one needs to check if struct serializes/deserializes
properly. This commit implements helpers for such cases including:
1. JSON
2. io.Serializable interface
add pingInterval same as used in ref C# implementation with the same logic
add pingTimeout which is used to check whether pong received. If not -- drop the peer.
add pingLimit which is hardcoded to 4 in TCPPeer. It's limit for unsuccessful ping/pong calls (where pong wasn't received in pingTimeout interval)
