snapstate: undo a daemon restart on classic if needed

[mvo5]

If we are on classic and have no previous version of core
we may have restarted from a distro package into the core
snap. We need to undo that restart.

This is essentially #3750 without the controversial integration test.

See also
https://forum.snapcraft.io/t/snapd-not-restarting-on-revert/1679

[zyga]

LGTM

[pedronis]

lgtm, log message could be clearer though I think

[pedronis]

undo classic initial core install?

[codecov-io]

Codecov Report

Merging #3757 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3757      +/-   ##
==========================================
+ Coverage   75.84%   75.85%   +<.01%     
==========================================
  Files         399      399              
  Lines       34541    34551      +10     
==========================================
+ Hits        26197    26207      +10     
+ Misses       6481     6480       -1     
- Partials     1863     1864       +1

Impacted Files	Coverage Δ
overlord/snapstate/handlers.go	65.1% <100%> (+0.31%)	⬆️
overlord/snapstate/snapstate.go	80.45% <0%> (-0.27%)	⬇️
cmd/snap/cmd_aliases.go	95% <0%> (+1.66%)	⬆️
interfaces/sorting.go	100% <0%> (+2.56%)	⬆️

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[zyga]

This failed iwth

----------------------------------------------------------------------

FAIL: picfg_test.go:124: piCfgSuite.TestConfigurePiConfigNoChangeSet

picfg_test.go:134:

    c.Check(st1.ModTime(), DeepEquals, st2.ModTime())

... obtained time.Time = time.Time{sec:63638909594, nsec:48592565, loc:(*time.Location)(0x7d4c80)} ("2017-08-21 10:53:14.048592565 +0000 UTC")

... expected time.Time = time.Time{sec:63638909594, nsec:52592533, loc:(*time.Location)(0x7d4c80)} ("2017-08-21 10:53:14.052592533 +0000 UTC")