raftexample: added build instruction and minor refactoring

[trung]

When trying out raftexample, there's no instruction on how to build the source. So i added few steps to make it clearer to the user.

Also using defer to unlock mutex in couple of places in kvstore.go.

[codecov-io]

Codecov Report

Merging #10496 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10496      +/-   ##
==========================================
- Coverage   71.57%   71.54%   -0.03%     
==========================================
  Files         392      392              
  Lines       36518    36518              
==========================================
- Hits        26136    26128       -8     
- Misses       8552     8556       +4     
- Partials     1830     1834       +4

Impacted Files	Coverage Δ
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-4.09%)	⬇️
etcdserver/api/v3rpc/watch.go	80.06% <0%> (-3.93%)	⬇️
etcdserver/util.go	95% <0%> (-3.75%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
etcdserver/api/v3election/election.go	66.66% <0%> (-2.78%)	⬇️
proxy/grpcproxy/watch.go	89.75% <0%> (-1.81%)	⬇️
etcdserver/api/v2http/client.go	85.51% <0%> (-1.21%)	⬇️
clientv3/op.go	73.93% <0%> (-0.86%)	⬇️
clientv3/leasing/kv.go	89.03% <0%> (-0.67%)	⬇️
... and 16 more

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[xiang90]

lgtm