implemented heartbeat timer for LibUV example

[thidas1290]

broker disconnects the client when heartbeats are not sent in the given LibUV example

• implemented a timer for sending heartbeats.

[EmielBruijntjes]

For LibEv this has already been implemented, but the code is much more complex over there. For example:

• It is possible for a single LibEv instance to monitor multiple connections (and thus have multiple timers active)
• What if the object is destructed before any heartbeat timers were activated?
• We also take into account that no heartbeats have to be sent if we already sent some other kind of data
• We also close the connection if the server failed to send a heartbeat in time
• We do some stuff with interval/2 to ensure that we're really on time with sending the heartbeat

Can you take a look at such things? Otherwise it might be better to just disable heartbeats.

[EmielBruijntjes]

Take a look at the LibEv implementation, which has much more details to send and process heartbeats. Is it possible to turn that class into a libuv implementation?

[thidas1290]

@EmielBruijntjes updated the LibUV heartbeat implementation referencing LibEv example such that,

• can have multiple connections, each connection having a heartbeat timer and multiple fd watchers.
• overall heartbeat sending logic remains untouched, changes are made only to time conversions and LibUv specific functions