Add history.exchange sub-package with following responsibilities
- fetch and caches daily exchange rates for tokens
- Partial update if missing form yesterday back
- Implement all time fetching special case
- Fetches the price of the token after merging entries for the selected
chains
history.service package changes
- Return ValuePoint instead of DataPoint
- Value point contains the value in fiat as float64 instead
- Convert merged values from tokens balance (wei) to fiat
Other changes
- Move start/stop balance history to startWallet/stopWallet
- Move cryptocompare to thirdparty package to avoid recursive import
- Rename DataPoint.Value to DataPoint.Balance
- Don't merge entries for single chain
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