Merge #13627: Free keystore.h from file scope level type aliases

Summary:
d0b9405f964670d6eaa8529f65fa7993b2a406c8 Refactors keystore.h type aliases. (251)

Pull request description:

This pull request frees `keystore.h` from type alias declarations that have been declared at file scope level.

`keystore.h` has various type aliases that have been declared ~3 - 6 years ago at file scope level, which can either be encapsulated or removed.

Where type alias declarations are encapsulated at the appropriate scope and access level, C++11's `using` notation is used in favor of the `typedef` notation.

Tree-SHA512: 1395cdc63e0c7ff5a1b1721675ad4416f71f507e999bd4ba019f03457cbfc08877848f10a8db7f5ccd2cd5ca3f5a291c986616f7703172fb6d79fba7447ffba8

Backport of Core PR13627
https://github.com/bitcoin/bitcoin/pull/13627/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4664