raft: prevent learners from becoming leader

[tbg]

We were already taking some precautions against learners campaigning,
but there was no safeguard against an explicit call to Campaign().
The newly added test also verifies that leadership transfers to
learners are ignored.

[codecov-io]

Codecov Report

Merging #10822 into master will decrease coverage by 0.18%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master   #10822      +/-   ##
==========================================
- Coverage   62.79%    62.6%   -0.19%     
==========================================
  Files         391      391              
  Lines       37318    37323       +5     
==========================================
- Hits        23433    23366      -67     
- Misses      12297    12359      +62     
- Partials     1588     1598      +10

Impacted Files	Coverage Δ
raft/raft.go	91.98% <60%> (-0.23%)	⬇️
client/client.go	48.03% <0%> (-35.95%)	⬇️
clientv3/ordering/kv.go	58.82% <0%> (-26.48%)	⬇️
clientv3/leasing/util.go	88.33% <0%> (-10%)	⬇️
etcdserver/api/v3rpc/watch.go	76.14% <0%> (-4.25%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-4.09%)	⬇️
pkg/transport/listener.go	51.18% <0%> (-3.8%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
clientv3/client.go	76.27% <0%> (-1.7%)	⬇️
proxy/grpcproxy/lease.go	78.28% <0%> (-0.91%)	⬇️
... and 19 more

---

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[xiang90]

lgtm

[gyuho]

/cc @jingyih