Improve the failover efficiency

[greatsharp]

This enhancement can improve the failover efficiency when a lots of master node probe failed concurrently.

Since there is only one can UpdateCluster successfully each time, we should reset the failure count after UpdateCluster successfully, and can do failover later again, there is no need to wait more than 15 seconds(ping_interval_seconds*max_ping_count).

For example, if 3 master node doing failover concurrently, it will cost more than 45 seconds before, but now it is 18 seconds (12s + 3s + 3s).

BTW, we should enlarge the minIdleConns to each node, this can save time in acquiring new connections when doing probe and sync cluster topology to all nodes, 10 is enough.

[git-hulk]

@greatsharp Thanks for your improvement.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.11%. Comparing base (6c56470) to head (0bfb1d2).
⚠️ Report is 97 commits behind head on unstable.

Files with missing lines	Patch %	Lines
controller/cluster.go	50.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #362      +/-   ##
============================================
+ Coverage     43.38%   47.11%   +3.72%     
============================================
  Files            37       45       +8     
  Lines          2971     4453    +1482     
============================================
+ Hits           1289     2098     +809     
- Misses         1544     2147     +603     
- Partials        138      208      +70     

Flag	Coverage Δ
unittests	47.11% <50.00%> (+3.72%)	⬆️

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