PostgreSQL: real type is "6 decimal digits precision"

[Kexkey]

https://www.postgresql.org/docs/current/datatype-numeric.html

Currently, watching_tx.amount, recipient.amount as well as tx.fee columns are of type real which causes precision loss when the value is > 1.

We need to change those types to double.

[Kexkey]

Let's move from floats to bigints for persistance and deal with decimal points in responses only...