server: Improve Raiden identifier

[ezdac]

The usage of Raiden identifier as unique secret tokens is flawed in two ways:

• first, it is in bruteforce reach, since 64 bit does not provide enough entropy
• second, the currently used SQLite DB backend does not support unsigned 64 bit integers natively - as a hotfix we restrict the int's to be signed and only chose values in the 63 bit positive range

https://github.com/raiden-network/paywall/blob/c84e5407f60e5316dd477ca4a9bbf89ad2749c00/server/src/raiden_paywall/flask_raiden.py#L382-L395