Add privacy to service credentials

[s-tikhomirov]

Service credentials (see #59 ) break the link between paying for the service and the service itself, which may ensure stronger client privacy.

In a credential-based scheme:

1. the client deposits funds into an on-chain pool;
2. the client generates a credential that proves the transfer in zero-knowledge;
3. the client sends the credential to the server;
4. the server uses the credential to pull funds from the pool.

Further reading: one, two, three.