Move config manager singleton to namespace

[ranisalt]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Using a namespace rather than a singleton/global has two major advantages:

• it avoids accidentally creating a new instance (which can be fixed with private constructor + getInstance static method, but this is verbose)
• it does not generate RTTI for the class

It also reduces the size of the header by hiding what would be private in an anonymous namespace, and gives better opportunity to optimize it (no external interface to respect = compiler can go nuts)

Issues addressed: