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.

[system]

db-driver   = postgres
db-host     = localhost
db-port     = 5433
adapter     = native
db-name     = cw_fti_test
db-encoding = UTF-8
db-user     = syt
db-password = syt

[admin]
login = admin
password = gingkow