Investigate if `messageCid` is needed as a param for `DataStore.get(...)` and remove if unnecessary

[thehenrytsai]

messageCid is mainly used for reference tracking for put() and delete(), it does not appear to be necessary at a glance for get(), and is there only to provide a "consistent look-and-feel" across methods but may not be worth it.

Investigate if messageCid in get() is really necessary and remove if not.