wal: allow non-WAL files in wal directory

[gyuho]

Fix #7287.

In other words, WAL directory with a single lost+found file will be considered not initialized.

[codecov-io]

Codecov Report

Merging #9743 into master will decrease coverage by 0.21%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #9743      +/-   ##
==========================================
- Coverage   69.56%   69.34%   -0.22%     
==========================================
  Files         374      375       +1     
  Lines       35115    35128      +13     
==========================================
- Hits        24427    24359      -68     
- Misses       8928     9010      +82     
+ Partials     1760     1759       -1

Impacted Files	Coverage Δ
pkg/fileutil/fileutil.go	61.11% <ø> (-4.11%)	⬇️
wal/util.go	80% <100%> (ø)	⬆️
pkg/fileutil/read_dir.go	91.3% <91.3%> (ø)
proxy/grpcproxy/register.go	69.44% <0%> (-11.12%)	⬇️
pkg/adt/interval_tree.go	79.87% <0%> (-8.71%)	⬇️
etcdserver/apply.go	82.36% <0%> (-6.66%)	⬇️
pkg/netutil/netutil.go	63.11% <0%> (-5.74%)	⬇️
raft/progress.go	94.17% <0%> (-1.95%)	⬇️
discovery/discovery.go	67.2% <0%> (-1.62%)	⬇️
raft/node.go	90.83% <0%> (-1.6%)	⬇️
... and 17 more

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