staking-info improvement on claimed

[Imod7]

Description

This is an issue in continuation of the changes made in the #1445 PR, to implement a performance improvement and test checking in the staking-info endpoint (claimed field).

Performance Improvement

In the staking-info endpoint, an improvement can be done regarding the calculation of claimed that can potentially increase the performance of the endpoint.

• Moving the OldCalls check outside the eras loop

Credits for this idea to @filvecchiato

Double Check Test Cases

It will be a good opportunity with the above improvement to also review existing tests & check if the below cases are covered:

• Case of era < 518 and chain = isKusama -> the era is considered as claimed "automatically".
• Case of status = undefined -> which is when we do not have enough info to determine the status of the era.
• Case where staking.erasStakersOverview = null & erasStakers = 0 which can be implemented by mocking this request http://127.0.0.1:8080/accounts/CmjHFdR59QAZMuyjDF5Sn4mwTgGbKMH2cErUFuf6UT51UwS/staking-info?at=22869643

If not, consider of adding them as tests or adding those cases in the STAKING_IMPLEMENTATION_DETAILS guide.