Convert GlobalCache to its own class

In general, Common.lua should be for functions that can be used on any table or number or other type. Many of these functions there are very specific to the GlobalCache. A few options I'd rather see

1. Make the functions more generic (e.g. for mergeDB here, have it be mergeTables and call it twice for conditions and multipliers)
2. Move the functions to a separate file specifically for managing the GlobalCache (might be worth turning GlobalCache into a proper OO class potentially)
3. Some of the functions are only used in one place. It might be better off having those functions in the file that uses them either as a local function or a function defined in that file instead of polluting the global namespace

Originally posted by @Wires77 in https://github.com/PathOfBuildingCommunity/PathOfBuilding/pull/2294#discussion_r602679854

Start by auditing the GlobalCache-specific functions in Common.lua and tracing their call sites. Compare the three proposed directions—generic helpers, a dedicated GlobalCache file or class, and localizing single-use functions—before choosing a design. Done means the responsibilities are separated without changing existing behavior, but this issue does not define a specific acceptance test.