[Keep-Alive] Server pinging

[analogrelay]

As part of keep-alive, we want the client to automatically consider a connection terminated if the server hasn't sent anything in a while. In order for that to work, the server needs to send Ping messages frequently. However, we only want to do this for the WebSockets and Server-Sent Events transports. For Long Polling, it would disrupt the "long poll" by ending it prematurely and is unnecessary since the transport already supports a "reconnection" mechanic for issues inherent to long polling.

Related: #1157

[analogrelay]

Additional Notes:

• The server will only ping if a message has not already been sent within the ping window. So a connection that is already sending data at the appropriate intervals will not require any ping frames.