Add constraints fore saving/reading files

[davidbrochart]

Problem

Currently the default (async) file manager allows to save a file and to read it concurrently. If this happens, the read file content will probably not consist of the whole file, but a part of it.
Also, since save() is async, it could be cancelled in the middle, which will likely corrupt the file.

Proposed Solution

I am wondering if writing a file should prevent reading it at the same time. Also, I think that writing a file should be shielded from cancellation.
I think these constraints should be enforced here in jupyter-server, since this would avoid lots of issues upstream when using these methods. We could for instance use a file path based locking mechanism for preventing reading-while-writing.

Additional context

jupyter-collaboration shields write operations here, and uses locks for preventing reading-while-writing.