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.

[MESSAGES CONTROL]
disable-msg = C0301

[VARIABLES]
# Enable / disable this checker
enable-variables = yes
additional-builtins = _, display_name

[BASIC]
required-attributes=
attr-rgx = (w|[a-z_][a-z0-9_]{2,30})
variable-rgx = (w|[a-z_][a-z0-9_]{2,30})$
argument-rgx = (w|[a-z_][a-z0-9_]{2,30})$

[TYPECHECK]
generated-members=debug,info,notice,warning,error,critical,exception