Encryptor fees and slots management in subscription

[vzotova]

Type of PR:

• [ ] Bugfix
• [X] Feature
• [ ] Documentation
• [ ] Other

Required reviews:

• [ ] 1
• [ ] 2
• [X] 3

What this does:

High-level idea of the changes introduced in this PR. List relevant API changes (if any), as well as related PRs and issues.

Issues fixed/closed: Refs #274

Why it's needed:

Explain how this PR fits in the greater context of the NuCypher Network. E.g., if this PR address a nucypher/productdev issue, let reviewers know!

Notes for reviewers: Based on #272