plex.GetDocumentAsync needs to use paging

[ukdtom]

When requesting a response from plex.GetDocumentAsync paging should be used, to avoid database lockup => PMS Crash, as well as cope for what the future might bring

[mfeingol]

PR also welcome. However, it does complicate things since the return value would no longer be a JsonDocument but an enumerable of JsonDocuments, or we'd have to push the paging mechanism upward to all callers.