(#15106) WatchedFile, treat absent files as unchanged

The watchedfile class was extracted from another location but had some
remnants of old behavior. In addition it would treat missing files as
always changed, which could lead to undesired behavior around trying to
reload missing files.

This commit refactors the WatchedFile to use a single timestamp value
and assume that the file is unchanged if the file timeout period has not
elapsed.

It also treats file absence as a valid state. If a file is missing
between two checks it will be treated as unchanged.