Fill it quicker when immediately needed, removing the need for a request list and all relating to it in the corresponding background service.
Additionally, updated usings, style, nullables in all affected files.
Implemented several workarounds to what appearto be a library bug in
which not all users will be downloaded onto the cache. User cache
checking and downloading is delegated to a new Common method and a
new BackgroundService, respectively.
This is hopefully temporary.
All existing VB code was 'translated' to C# as closely as possible, with
minor changes and additional notes. Currently untested and likely
broken.
Further commits will go toward making overall improvements until this
version replaces the currently existing code.
