Heartbeat calls: new feature and optimizations

[crimsonred]

Reduce heartbeats
Selective heartbeat to channels

[codecov-io]

Codecov Report

Merging #56 into master will decrease coverage by 1.58%.
The diff coverage is 56.68%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
- Coverage   82.81%   81.22%   -1.59%     
==========================================
  Files          37       39       +2     
  Lines        3829     3942     +113     
==========================================
+ Hits         3171     3202      +31     
- Misses        580      655      +75     
- Partials       78       85       +7

Impacted Files	Coverage Δ
presence_request.go	0% <0%> (ø)
pubnub.go	80.1% <28.57%> (-2.12%)	⬇️
heartbeat_manager.go	75.18% <75.18%> (ø)
subscription_manager.go	75.42% <80%> (-2.75%)	⬇️
state_manager.go	66.66% <0%> (-3.51%)	⬇️

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