Replaced HasStoredState with IsSyncedWithNetwork

[rawfalafel]

See this comment for more background.

[codecov]

Codecov Report

Merging #662 into master will increase coverage by 0.8%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #662     +/-   ##
=========================================
+ Coverage   74.14%   74.94%   +0.8%     
=========================================
  Files          55       55             
  Lines        3508     3497     -11     
=========================================
+ Hits         2601     2621     +20     
+ Misses        682      653     -29     
+ Partials      225      223      -2

Impacted Files	Coverage Δ
beacon-chain/db/state.go	75% <ø> (+17.85%)	⬆️
beacon-chain/sync/service.go	66.45% <100%> (+4.03%)	⬆️
beacon-chain/sync/initial-sync/service.go	77.27% <100%> (+14.11%)	⬆️
beacon-chain/rpc/service.go	77.29% <0%> (+0.96%)	⬆️

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[terencechain]

lgtm