wal: remove wal directory on windows before renaming tmpdir

[amdprophet]

Renaming on windows will not work if the destination already exists. This change removes the wal directory before the wal tmpdir is renamed to wal.

This has resolved an issue we had with embedded etcd failing to start. I'm not aware of any consequences of this change but please let me know if I'm missing something!

[heyitsanthony]

Won't this clobber data if two etcd processes launch on the same member directory concurrently?

[codecov-io]

Codecov Report

Merging #7915 into master will increase coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #7915      +/-   ##
=========================================
+ Coverage   75.62%   75.8%   +0.17%     
=========================================
  Files         336     336              
  Lines       26390   26390              
=========================================
+ Hits        19957   20004      +47     
+ Misses       4996    4948      -48     
- Partials     1437    1438       +1

Impacted Files	Coverage Δ
pkg/fileutil/purge.go	73.68% <0%> (-7.9%)	⬇️
proxy/grpcproxy/watch.go	90.97% <0%> (-3.48%)	⬇️
lease/leasehttp/http.go	62.59% <0%> (-1.53%)	⬇️
rafthttp/peer.go	90.07% <0%> (-1.53%)	⬇️
etcdmain/etcd.go	44.2% <0%> (-1.29%)	⬇️
clientv3/balancer.go	93.98% <0%> (-1.1%)	⬇️
rafthttp/stream.go	88.59% <0%> (-1.01%)	⬇️
clientv3/lease.go	91.57% <0%> (-0.77%)	⬇️
etcdserver/server.go	80.09% <0%> (-0.58%)	⬇️
raft/node.go	89.73% <0%> (-0.45%)	⬇️
... and 13 more

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[amdprophet]

@heyitsanthony If that is the case, wouldn't Rename end up with the same result if it had worked? Think mv on Linux and how it replaces the destination.

wal_unix.go is also already doing this. https://github.com/coreos/etcd/blob/master/wal/wal_unix.go#L33-L35

[heyitsanthony]

@amdprophet the unix one should be safe because it has the lock in the directory; windows doesn't

[heyitsanthony]

some more testing around this path is probably warranted; etcd probably shouldn't be clobbering wal directories unless it knows that it has exclusive access and nothing else is using it

[heyitsanthony]

Superseded by #8286. Closing