v3: gateway: Reconnect after heartbeat misses

[diamondburned]

Currently, the gateway does not try to reconnect if it detects that the gateway's last heartbeat ACK was behind time. This is an issue when the computer is suspended for a long period of time, but it might also be an issue in machines with spotty network conditions.

Ideally, utils/ws should handle the check, so we can have something like:

 type Handler interface {
        // OnOp is called by the gateway event loop on every new Op. If the returned
        // boolean is false, then the loop fatally exits.
-       OnOp(context.Context, Op) (canContinue bool)
+       OnOp(context.Context, Op) (heartbeat time.Time, canContinue bool)
        // SendHeartbeat is called by the gateway event loop everytime a heartbeat
        // needs to be sent over.
        SendHeartbeat(context.Context)

or

@@ -122,6 +122,8 @@ type Handler interface {
        // OnOp is called by the gateway event loop on every new Op. If the returned
        // boolean is false, then the loop fatally exits.
        OnOp(context.Context, Op) (canContinue bool)
+       // LastBeat returns the last heartbeat time.
+       LastBeat() time.Time
        // SendHeartbeat is called by the gateway event loop everytime a heartbeat
        // needs to be sent over.
        SendHeartbeat(context.Context)

The first method wouldn't require re-locking the state mutex after each loop iteration, so it seems like the better solution to me. Overloading the return type might be kind of weird, though.

[diamondburned]

660d989 introduced a regression.