Make instance variables in Frame private

[asomov]

Refactor: make the instance variables private and use getters

Types of Changes

• [x ] Cosmetics (whitespace, appearance)

Checklist

• [ x] I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• [x ] All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

[michaelklishin]

Thank you. Please note that this can only go into 6.0.