static-file: properly set/use cache header for static file (closes #2255013)
This changesets enables the standard http cache mechanism where the static
controller may reply "304 Not modified" based on `last-modified` in HTTP
response and `if-modified-since` in HTTP query. The last modified time is
computed using the file-system information.
The pre-existing logic using an `Expires` header to prevent client from sending
request stay in place. The new logic just prevents sending the file again if not
necessary.
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"""common to all configuration
"""