wal: use path/filepath instead of path

[tessr]

Use the path/filepath package instead of the path package. The
path package assumes slash-separated paths, which doesn't work
on Windows. But path/filepath manipulates filename paths in a way
that's compatible across OSes.

[codecov-io]

Codecov Report

Merging #7515 into master will increase coverage by 0.04%.
The diff coverage is 90%.

@@            Coverage Diff            @@
##           master   #7515      +/-   ##
=========================================
+ Coverage   70.25%   70.3%   +0.04%     
=========================================
  Files         320     320              
  Lines       26203   26203              
=========================================
+ Hits        18408   18421      +13     
+ Misses       6338    6332       -6     
+ Partials     1457    1450       -7

Impacted Files	Coverage Δ
wal/file_pipeline.go	75% <100%> (ø)	✅
wal/repair.go	48% <100%> (ø)	✅
wal/wal.go	61.81% <87.5%> (ø)	✅
auth/simple_token.go	86.84% <0%> (-7.9%)	❌
mvcc/watcher.go	95.65% <0%> (-4.35%)	❌
pkg/adt/interval_tree.go	78.71% <0%> (-3.72%)	❌
pkg/testutil/recorder.go	66.66% <0%> (-3.71%)	❌
pkg/netutil/netutil.go	77.94% <0%> (-2.95%)	❌
clientv3/watch.go	89.63% <0%> (-2.85%)	❌
clientv3/maintenance.go	67.6% <0%> (-2.82%)	❌
... and 14 more

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[heyitsanthony]

huh, it's managed to work on windows with path.Join but this is definitely the right thing to do. lgtm. Thanks!

[tessr]

Thanks for the quick merge! And it actually wasn't working for me on Windows (Windows 10, in a VM) until I made this change 😞

[xiang90]

@tessr Do you mind adding Chain to our noticeable raft user list here: https://github.com/coreos/etcd/tree/master/raft#notable-users?

[tessr]

@xiang90 sure thing! Will do soon.