*: fix the backendnum statistics error #698

[zhyass]

[summary] The slave node of Radon will synchronize metadata from the master node. At this time, backend.json will be reloaded, causing backendnum to be counted repeatedly.

Set 0 before reload the backend config.

[test case] src/monitor/monitor_test.go

[patch codecov] src/monitor/monitor.go 96.3% src/backend/scatter.go 88.7%

[codecov[bot]]

Codecov Report

Merging #699 into master will decrease coverage by 0%. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #699   +/-   ##
=====================================
- Coverage      89%    89%   -1%     
=====================================
  Files         130    130           
  Lines        9279   9281    +2     
=====================================
- Hits         8269   8267    -2     
- Misses        669    671    +2     
- Partials      341    343    +2     

Impacted Files	Coverage Δ
src/backend/scatter.go	81% <100%> (+<1%)	:arrow_up:
src/monitor/monitor.go	93% <100%> (+<1%)	:arrow_up:
src/executor/engine/merge_join.go	95% <0%> (-2%)	:arrow_down:
src/backend/txn.go	87% <0%> (-1%)	:arrow_down:

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