server: add parameter semi-sync-timeout-for-two-nodes

[codecov[bot]]

Codecov Report

Merging #131 (5160bc9) into master (60b1efc) will increase coverage by 0%. The diff coverage is 96%.

@@          Coverage Diff          @@
##           master   #131   +/-   ##
=====================================
  Coverage      67%    68%           
=====================================
  Files          48     48           
  Lines        3740   3742    +2     
=====================================
+ Hits         2541   2547    +6     
+ Misses        963    960    -3     
+ Partials      236    235    -1     

Impacted Files	Coverage Δ
src/raft/leader.go	84% <0%> (ø)
src/config/config.go	80% <100%> (+<1%)	:arrow_up:
src/raft/mock.go	83% <100%> (ø)
src/raft/raft.go	84% <100%> (+<1%)	:arrow_up:
src/server/server.go	51% <100%> (ø)
src/raft/candidate.go	83% <0%> (+2%)	:arrow_up:

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[hustjieke]

LGTM.