Improve GetRemoteStatus and db.GetStatus() logic

[NyaaaWhatsUpDoc]

• removes chance of db.GetStatus() recursion
• improves GetRemoteStatus() to require specifying if parent / children are required in status
• fix StatusCache to return object copies, and only store information about status itself (not related attachments)
• set cache=shared by default on SQLite instances to ensure thread-safety between operations (this was the source of the nested transaction related errors we were seeing). why this was not set as a default in a Go library where you have to go out of your way to run single-threaded?! who knows

[NyaaaWhatsUpDoc]

Snuck in another bugfix with a panic caused by EnrichRemoteAccount :)

[tsmethurst]

Just the testing stuff needs to change, the rest looks great 👍