Better pings 2

[cadorn]

This pull-request cleans up the commits for #78.
For previous discussion see: #78

[cadorn]

Also, since you're now sending the pingInterval in the handshake we need to update SPEC in the engine.io-client repo and bump up the revision to 2, since clients will be rendered incompatible.

Thinking about this reminded me of an observation I made when writing this patch.

Why are pings initiated by the server?

This patch provides the following:

server on `pingInterval` send `ping` and expect `pong` within `pingTimeout` or emit `ping timeout`
client on `ping` send `pong` and expect ack `pong` within `pingTimeout` or emit `ping timeout`
client expect `ping` within every `pingInterval + pingTimeout` or emit `ping timeout`

By necessity to accomplish pingInterval + pingTimeout timeout on client, server must send an ack pong packet (this is new). Instead of the ack pong packet we cloud look for clean transport request close after sending pong packet on client but that does not ensure server is getting our responses.

If pings were initiated by client we could get rid of the ack pong packet (good especially for polling) and still timeout within pingInterval + pingTimeout on client:

client on `pingInterval` send `ping` and expect `pong` within `pingTimeout` or emit `ping timeout`
server on `ping` send `pong` and expect next `ping` within `pingInterval + pingTimeout`

This would mean more changes to the SPEC and client implementations but is a cleaner and lighter lifecycle. I am leaning towards this approach as I typically always initiate pings on clients.

[rauchg]

Sounds good, since we're changing pings we might as well switch to that. Good point @cadorn

[cadorn]

Done here & socketio/engine.io-client#51

All tests still pass! (other than two minor adjustments)

Now only need to update SPEC.

[rauchg]

@cadorn just merged this, looking good. Make sure to submit a PR for SPEC.

[rauchg]

Thanks a lot!

[cadorn]

See #115 for spec.