(see what I did there? 🤭) … and raise PHPStan Level to 5

This is a follow-up to #151, where I naively updated the connection class itself.

This PR introduces the following changes on the Connection class. It proposes…

• renaming the startHeathbeatTimer() method to startHeartbeatTimer() (and the related call in the Client class)
• removing the unreachable condition that I targeted in Remove unreachable condition and bring PHPStan to Level5 #151, this time in the right place
• replacing a long one-liner with $a = null; $b = null; $c = null; ... to $a = $b = $c = null;, to make it a little bit shorter
• adding some return types
• using null coalescing instead of isset() checks