fix for storing uint64 values as int64 in db

[pk910]

Dora fails and gets stuck when trying to store uint64 values with the highest bit set into db.
This was previously not a problem, but with withdrawal requests there is now a way to put arbitrary numbers into requests, so dora needs to be prepared to handle those high values.

The background of this limitation is, that values are stored as signed 64 bit number in db, so it can only handle the lower half of the uint64 range.
To get around that limitation, the uint64 values are now converted to the int64 range for storing in db,
That means:
uint64(0) -> int64.MinValue
uint64.MaxValue -> int64.MaxValue

With this value translation, we can store big uint64 to the db and still filter based on min/max values, as the order of values within the range keeps stable.