optimize transfer leader

[absolute8511]

Optimize the cluster change to avoid read/write on non-ready raft node. This can reduce some errors while cluster is changed by network or node restarting.

[codecov]

Codecov Report

Merging #32 into master will decrease coverage by 0.23%.
The diff coverage is 38.03%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   48.02%   47.78%   -0.24%     
==========================================
  Files         104      103       -1     
  Lines       17351    17491     +140     
==========================================
+ Hits         8332     8358      +26     
- Misses       7665     7771     +106     
- Partials     1354     1362       +8

Impacted Files	Coverage Δ
common/util.go	0% <ø> (ø)	⬆️
common/type.go	23.84% <ø> (ø)	⬆️
transport/rafthttp/peer.go	75.57% <ø> (-1.53%)	⬇️
cluster/pdnode_coord/place_driver.go	13.6% <0%> (ø)	⬆️
cluster/pdnode_coord/pd_coordinator.go	0% <0%> (ø)	⬆️
wal/decoder.go	88.37% <100%> (ø)	⬆️
transport/rafthttp/http.go	78.12% <100%> (+2.51%)	⬆️
server/httpapi.go	14.1% <16.66%> (+0.06%)	⬆️
node/namespace.go	36.66% <18.75%> (-1.05%)	⬇️
wal/wal.go	57.97% <23.07%> (-2.35%)	⬇️
... and 8 more

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