Initial cleanup for cluster refactoring

[madolson]

Cleaned up the minor cluster refactoring notes that were intended to be follow ups that never happened. Basically:

1. Minor style nitpicks
2. Generalized clusterNodeIsMyself so that it wasn't implementation dependent.
3. Removed getMyClusterId, and just make it an explicit call to myself's name, which seems more straightforward and removes unnecessary abstraction.
4. Remove clusterNodeGetSlaveof infavor of clusterNodeGetMaster. We already do a check if it's a replica, and if it wasn't working it would have been crashing.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 68.88%. Comparing base (cde8ec1) to head (de545a9).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #460      +/-   ##
============================================
- Coverage     68.91%   68.88%   -0.03%     
============================================
  Files           109      109              
  Lines         61793    61789       -4     
============================================
- Hits          42582    42565      -17     
- Misses        19211    19224      +13     

Files	Coverage Δ
src/cluster.c	83.03% <75.00%> (+0.29%)	:arrow_up:
src/cluster_legacy.c	83.24% <0.00%> (+0.07%)	:arrow_up:
src/module.c	9.34% <0.00%> (ø)

... and 14 files with indirect coverage changes