server: started ShardServer

- protocol: added GenSerialKey() which securely generates an EnterSerialKey
- login server accepts a shard via LoginServer.AddShard()
- login server will pass LoginMetaData to the selected shard via ShardServer.QueueLogin()
- misc. refactoring

server/shared.go

@@ -0,0 +1,9 @@
+package server
+
+import (
+	"github.com/CPunch/gopenfusion/protocol"
+)
+
+type PacketHandler func(peer *Peer, pkt protocol.Packet) error
+
+func stubbedPacket(_ *Peer, _ protocol.Packet) error { /* stubbed */ return nil }