We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
Before this patch, when you called FileSystemAccessServer::Client::try_*
twice, the second call used the same variable to store the promise. This
"race condition" is now solved using a HashMap, to store multiple
parallel requests.
