Investigate float conversion issue in go1.14-arm64 and replace workaround.

I created a workaround in #20764 in order to avoid what looks like a conversion bug in go1.14-arm64 from `float64` to `uint64`. This yielded [this monstrosity](https://github.com/ethereum/go-ethereum/blob/master/les/utils/expiredvalue.go#L98) which has to be investigated further.