Simplify isNodeAvailable function and add comments

[enjoy-binbin]

Use getNodeReplicationOffset to replace the original logic and add the corresponding annotations.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.09%. Comparing base (9033734) to head (a3faef3). Report is 367 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/cluster_legacy.c	50.00%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #994      +/-   ##
============================================
+ Coverage     70.57%   71.09%   +0.51%     
============================================
  Files           114      123       +9     
  Lines         61634    65531    +3897     
============================================
+ Hits          43498    46588    +3090     
- Misses        18136    18943     +807     

Files with missing lines	Coverage Δ
src/cluster.c	89.18% <100.00%> (+0.80%)	:arrow_up:
src/cluster_legacy.c	86.10% <50.00%> (+0.14%)	:arrow_up:

... and 109 files with indirect coverage changes

[enjoy-binbin]

@zuiderkwast this is a small cleanup, do you want to take a look?