Pass interface to Client + other improvements

[klaidliadon]

This PR does the following:

• allows to pass a proto.QuotaControl to the NewClient function: if it's nil, it is going to behave as usual by creating a new client using the configuration, otherwise it's going to use the passed instance which could be the server implementation, for instance.
• Adds a new service to the enum
• Adds a new SessionType User, that is enabled by passing a UserStore. When there's a Wallet session type, it uses the store to lookup for the user. If a user is found the session type gets upgraded from Wallet to User
• Changes the ACL to have more flexibility. Instead of only a minimum level, a series of levels can be specified. The ACL is now an integer encodes the level as bits (Public is bit 0, AccessKey is bit 1 and so on)

[VojtechVitek]

LGTM