session-helper: refactor creation and updating of real path monitors

I think this enhances readability significantly, reduces code duplication
and allows you to follow the rationale behind why monitors are being
added/removed a lot more clearly than the previous nested/ad-hoc logic in
file_changed. Also adds debug printouts in the case that file monitors
are not created successfully.

Closes: #1194
Approved by: pwithnall
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