Large numbers can be returned by the Flexpool API reaching the `int64`
limit. Blocks reward were seen as 0 ETH. Using `float64` instead. Also
converting signed int64 to unsigned int64 for non-timestamp numbers.
BREAKING CHANGE: please follow the upgrades instructions from 1.2 to
1.3 to convert such types in the database.
Signed-off-by: Julien Riou <julien@riou.xyz>
