go/consensus/tendermint/apps/staking: Fix epochtime overflow

[kostko]

Based on #2630
Fixes #2627

TODO

• Add test for overflow in double signing evidence handler.
• Fix overflow in double signing evidence handler.
• Check if there are other places which overflow epochtime.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@8b88a33). Click here to learn what that means.
The diff coverage is 62.5%.

@@            Coverage Diff            @@
##             master    #2631   +/-   ##
=========================================
  Coverage          ?   63.05%           
=========================================
  Files             ?      360           
  Lines             ?    34142           
  Branches          ?        0           
=========================================
  Hits              ?    21529           
  Misses            ?     9953           
  Partials          ?     2660

Impacted Files	Coverage Δ
go/consensus/tendermint/apps/registry/registry.go	64.35% <0%> (ø)
go/consensus/tendermint/apps/staking/staking.go	61.66% <0%> (ø)
go/consensus/tendermint/apps/staking/slashing.go	73.33% <100%> (ø)

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