Fix isSystemPacket, refactor net code.

[Raugharr]

This PR does the following:

1. The isSystemPacket check never worked as the PingResolver never explicitly checked if the packet was a system packet, causing the first registered packed to be interpreted as a ping.
2. Removes one of the Connection constructors that would partially setup the connection then finish setting up with onConnect. Instead create the Connection add listeners, then connect with connect. This also removes the onConnect method and puts it into the connect method.
3. Add CloseConnection A packet that declares the other side is disconnecting.
4. Moves the CallbackResolver code into the AbstractResolver so you can do callbacks on every resolver now.
5. More comments and debug logs.

[sandysimonton]

Unused imports
Is Type ever going to be needed outside AbstractPacket
Can we name the nested interface "PacketType" instead, to disambiguate from other Types
Maybe getType instead of getId?

[sandysimonton]

Do we want to gracefully handle this?

[Raugharr]

Do you have any suggestions? Currently the hope is if someone disconnects, the other end will know either because the CloseConnectionResolver gets called and closes the connection, or the heartbeat doesn't get a response and it closes that way.

[sandysimonton]

So sorry I put comment on wrong line, should be line 50

throw new InvalidPacketException("Invalid PacketType");

Having this in mekwars/common might be ok... Do all modules know they have to handle the InvalidPacketException?

I guess my concern was that the InvalidPacketException is thrown and crashes the client or server because it propagates to the top of the stack

[Raugharr]

Yes, these methods only get called by Connection#readObject, which only gets called by Connection#read, which fizzles any packet that doesn't have a valid packet type.

[sandysimonton]

At your discretion