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

まず Common.lua にある GlobalCache 固有の関数を監査し、それらの呼び出し箇所を追跡します。設計を選択する前に、提案されている3つの方向性—汎用ヘルパー、専用の GlobalCache ファイルまたはクラス、単一用途の関数のローカライズ—を比較します。既存の動作を変更せずに責務が分離されていれば完了ですが、この issue では具体的な受け入れテストは定義しません。