Refactor(StakeManager): Abstract out part of the contract that can be reused

[3esmit]

StakeManager.sol currently holds too much functionality.

For better code reuse, and readability, it should be split into separate contract whenever is possible.

Some obvious changes are:

• Place the storage contract on its own file
• Abstract the codehash verification logic to its own contract and inherit it from where its needed

Other possible changes are:

• Create library for dealing with mathematical formulas of APY
• Create library for dealing with account management
• Create library for dealing with epoch management
• (...) TBD

This issue will be closed once we are satisfied with the abstraction. We should experiment alternatives to see what we can improve in the code readability and reusability, considering the upcoming implementation of other staking solutions in this repository.