core/state/snapshot: fix data race in diff layer

[ucwong]

No description provided.

[codecov]

Codecov Report

Merging #927 (1dad6ec) into master (366f2b7) will decrease coverage by 0.03%.
The diff coverage is 20.00%.

❗ Current head 1dad6ec differs from pull request most recent head 8c649b2. Consider uploading reports for the commit 8c649b2 to get more accurate results

@@            Coverage Diff             @@
##           master     #927      +/-   ##
==========================================
- Coverage   49.47%   49.44%   -0.04%     
==========================================
  Files         415      415              
  Lines       46566    46572       +6     
==========================================
- Hits        23037    23026      -11     
- Misses      21535    21539       +4     
- Partials     1994     2007      +13     

Impacted Files	Coverage Δ
core/state/snapshot/difflayer.go	71.01% <20.00%> (-1.13%)	⬇️
rpc/server.go	71.05% <0.00%> (-5.27%)	⬇️
p2p/simulations/mocker.go	29.89% <0.00%> (-2.07%)	⬇️
p2p/discover/table.go	82.48% <0.00%> (-1.83%)	⬇️
rpc/client.go	83.52% <0.00%> (-1.18%)	⬇️
ctxc/downloader/downloader.go	75.37% <0.00%> (-1.17%)	⬇️
trie/proof.go	72.16% <0.00%> (-0.98%)	⬇️
trie/trie.go	78.24% <0.00%> (-0.77%)	⬇️
p2p/simulations/http.go	66.02% <0.00%> (-0.65%)	⬇️
p2p/server.go	65.09% <0.00%> (-0.43%)	⬇️
... and 6 more