Implement TCP connections

[jmaralo]

Implement the TCP logic. There are two main parts to it: the TCPSource and the TCPConn.

TCPSource exists because for each board we can either establish a connection as a server or as a client. The struct TCPClient is a TCPSource for single direct connections and the struct TCPServer for multiple connections. This allows to abstract the "establishing connection" from the "using connection logic". Sources run and notify when a connection is made through callbacks.

TCPClient retries to reconnect if the connection fails using an exponential backoff. Once the connection is created, TCPConn is in charge of notifying disconnections and external logic should handle calling again TCPClient.Run()

TCPServer creates a TCP listener and accepts connections specified on a list.

TCPConn allows to read and write from the connection, it also provides a callback to notify when the connection status changes (sends true when connected and false when disconnected).