Implement multiple master wallets for managed user wallets

[ygrishajev]

Currently managed wallets are created as child ones from a single master wallet. It is also used to grant spending as a part of our billing pipeline. We need to be able to use multiple master wallets for various usecases.

Some of them are as follows:

• split authz for trialing and paying users
• improve regular jobs performance by splitting authz grants between multiple wallets

This issue purpose is design the changes and come up with a proposal.

[lechenghiskhan]

Ideally, we should seriously consider implementing size limits per master wallet for security purposes. For example: no master wallet can exceed $100k (configurable) in outstanding authorized funds at any point. We'll probably have to create new master wallets as needed and/or create a few in reserve to handle scaling.