Create worker thread for file message slicing & encryption

[MathJud]

When a larger file is sent from libqaul, the file needs to be sliced into the maximally allowed messages size, and each message needs to be encrypted. For larger files this can take quite a while.

To speed up the process and not block libqaul for too long, move the file slicing and encryption into an async worker thread.

The thread should be created here:

qaul.net/rust/libqaul/src/services/chat/file.rs

Line 520 in 899edac

// TODO: start in new async thread here

[MathJud]

This issue is worked on in PR #550