*: use 'io.Seek*' for go1.7+

[gyuho]

For #6174.

[xiang90]

LGTM

[gyuho]

@heyitsanthony This breaks in jenkins. When do we switch to go 1.8 in jenkins?
We can just wait until then.

[gyuho]

Nuked the time.Until change and keeping go1.8 notes in README for later changes.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3a1368d). Click here to learn what that means.
The diff coverage is 44.44%.

@@            Coverage Diff            @@
##             master    #7545   +/-   ##
=========================================
  Coverage          ?   70.27%           
=========================================
  Files             ?      320           
  Lines             ?    26214           
  Branches          ?        0           
=========================================
  Hits              ?    18422           
  Misses            ?     6343           
  Partials          ?     1449

Impacted Files	Coverage Δ
pkg/fileutil/lock_linux.go	76.66% <ø> (ø)
pkg/fileutil/preallocate.go	25% <0%> (ø)
wal/repair.go	48% <0%> (ø)
pkg/fileutil/fileutil.go	69.56% <100%> (ø)
wal/encoder.go	84.31% <100%> (ø)
etcdctl/ctlv3/command/snapshot_command.go	64.77% <25%> (ø)
wal/wal.go	61.81% <50%> (ø)

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