Get/Set Metadata is Hard to Reason About

[biniona]

As noted in a code review (#49 (comment)), it is confusing to reason about how the sync/async _[a]get_by_metadata code paths. __contains__ only uses the synchronous _get_by_metadata as it does not need ever intiailize objects, where as aget/get need to be conscious of which _aget_by_metadata method they use.

A possible refactor could be to convert _get_by_metadata and _aget_by_metadata into the following methods:

• _get_by_metadata - only gets by metadata if present, never sets by metdata
• _set_by_metatdata` - only sets metadata for synchronous objects, throws on async objects
• aysnc _aset_by_metadata - sets metadata for async objects or sync objects using asyncio.to_thread

This would be a clearer split between what code paths need to be split into async and sync (creating new objects) and what code paths can be shared between async / sync code (getting / checking for existence of objects).