*: fix the bug loadbalance does not take effect in streaming fetch st…

[zhyass]

…atements #687

[summary]
loadbalance = 1, select /*+ streaming */ need load balance to the replica node.

[test case]
src/backend/txn_test.go
[patch codecov]
src/proxy/execute.go 87.8%

[codecov]

Codecov Report

Merging #688 into master will decrease coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #688     +/-   ##
=======================================
- Coverage      89%    89%     -1%     
=======================================
  Files         130    130             
  Lines       10540   9279   -1261     
=======================================
- Hits         9482   8268   -1214     
+ Misses        711    669     -42     
+ Partials      347    342      -5     

Impacted Files	Coverage Δ
src/proxy/execute.go	82% <100%> (-1%)	⬇️
src/backend/txnmgr.go	78% <0%> (-7%)	⬇️
src/ctl/v1/peer.go	75% <0%> (-6%)	⬇️
src/ctl/v1/meta.go	85% <0%> (-5%)	⬇️
src/executor/executor.go	57% <0%> (-5%)	⬇️
src/ctl/v1/explain.go	51% <0%> (-5%)	⬇️
src/plugins/plugin.go	78% <0%> (-4%)	⬇️
src/executor/engine/common.go	72% <0%> (-4%)	⬇️
src/proxy/disk.go	76% <0%> (-4%)	⬇️
src/syncer/syncer.go	75% <0%> (-4%)	⬇️
... and 121 more

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