only roll back light sync if not fully validating

[ucwong]

No description provided.

[codecov]

Codecov Report

Merging #760 into master will increase coverage by 0.04%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #760      +/-   ##
==========================================
+ Coverage   50.33%   50.38%   +0.04%     
==========================================
  Files         409      409              
  Lines       52934    52935       +1     
==========================================
+ Hits        26645    26670      +25     
+ Misses      24277    24244      -33     
- Partials     2012     2021       +9     

Impacted Files	Coverage Δ
ctxc/downloader/downloader.go	78.58% <87.50%> (+0.02%)	⬆️
ctxc/sync.go	60.00% <0.00%> (-1.30%)	⬇️
whisper/whisperv6/whisper.go	54.00% <0.00%> (-1.22%)	⬇️
ctxc/downloader/statesync.go	57.89% <0.00%> (-1.06%)	⬇️
core/tx_pool.go	76.98% <0.00%> (-0.80%)	⬇️
p2p/peer.go	79.45% <0.00%> (-0.46%)	⬇️
p2p/server.go	66.98% <0.00%> (+0.38%)	⬆️
p2p/simulations/http.go	70.10% <0.00%> (+0.54%)	⬆️
trie/proof.go	77.61% <0.00%> (+1.11%)	⬆️
p2p/rlpx.go	71.38% <0.00%> (+1.41%)	⬆️
... and 3 more