LOGBACK-350 Prevent SocketNode constructor blocking due to lack of data from client

The constructor creates an ObjectInputStream, and this blocks until the stream
header is received from the client.

If the client doesn't send a stream header, construction of the SocketNode will
hang, preventing SimpleSocketServer from accepting new clients.

This change moves the construction of the ObjectInputStream into
SocketNode.run. As SimpleSocketServer runs this method in a new thread, a rogue
client won't prevent subsequent ones being able to connect.