refactor: Use struct to parse API responses

- Return bytes for each request to the Flexpool API
- Delegate JSON marshalling the higher functions
- Use int64 when possible

BREAKING CHANGE: database structure has to be updated to use integer
instead of real. Please follow instructions to upgrade from 1.0 to 1.1.

Signed-off-by: Julien Riou <julien@riou.xyz>

main.go

@@ -132,7 +132,7 @@ func main() {
 				log.Warnf("Could not fetch unpaid balance: %v", err)
 				continue
 			}
-			log.Debugf("Unpaid balance %.0f", balance)
+			log.Debugf("Unpaid balance %d", balance)
 			miner.Balance = balance
 			if miner.Balance != dbMiner.Balance {
 				dbMiner.Balance = balance