Make glz::async_map copyable and movable

[stephenberry]

DO NOT MERGE YET. MUST DETERMINE IF THE MAP ITSELF SHOULD BE SHARED OR JUST THE ELEMENTS.

// The async_map is copyable, but it only performs shallow copies
// This is to allow values like `std::atomic<int>` (and other thread safe types) to be held
// So, copying an async_map will result in a shared reference to the data
// In a multi-threading context this is often the desired behavior as thread safe logic
// is typically attempting to shared state across threads