MEDIUM: lua: Don't filter exported fetches and converters

MEDIUM: lua: Don't filter exported fetches and converters

Thanks to previous commits, it is now safe to use from lua the sample fetches
and sample converters that convert arguments, especially the strings
(ARGT_STR). So now, there are all exported to the lua. They was filtered on the
validation functions. Only fetches without validation functions or with val_hdr
or val_payload_lv functions were exported, and converters without validation
functions.

This patch depends on following commits :

  * aec27ef44 "BUG/MINOR: lua: Duplicate lua strings in sample fetches/converters arg array"
  * fdea1b631 "MINOR: hlua: Don't needlessly copy lua strings in trash during args validation"

It must be backported as far as 2.1 because the date() and http_date()
converters are no longer exported because of the filter on the validation
function, since the commit ae6f125c7 ("MINOR: sample: add us/ms support to
date/http_date)".