[Bug]: Fix memory leak

[tmolitor-stud-tu]

WE seem to have a smaller memory leak in our core.
That doesn't impact the main app process, but the push app extension process is heavily memory constrained ~40MiB and having a memory leak there can result in crashes (kills by iOS) of the push app extension.

I'm not sure, where this comes from. Currently my best guess would be the NSXMLParser by Apple, used by Monal to parse the incoming XML stream into MLXMLNode instances.

Since that NSXMLParser crashes occasionally, too, maybe we could replace it by a rust-based parser.
Replacing it does not guarantee to fix the memory leak, though.