Save config file after replica migration and brocast a PONG when role changed

[enjoy-binbin]

This does some cleanup around clusterSetPrimary:

1. In replica migration, the replica already has a new primary, we
   need to update the config file.
2. When the role changed, we need to broadcast a PONG in the cluster
   to notify the change. This is a follow up for Brocast a PONG to all node in cluster when role changed #1295.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.13%. Comparing base (110dd55) to head (d875efc).
Report is 53 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1677      +/-   ##
============================================
+ Coverage     71.05%   71.13%   +0.07%     
============================================
  Files           121      123       +2     
  Lines         65243    65611     +368     
============================================
+ Hits          46358    46670     +312     
- Misses        18885    18941      +56     

Files with missing lines	Coverage Δ
src/cluster_legacy.c	86.05% <100.00%> (+0.12%)	⬆️

... and 32 files with indirect coverage changes