ipfix-export: refactor ipfix_exporter_t_handler fn

Split this api handler into 2 parts. The first is the top level handler
and the second is the internal helper function that does all the work.
This is in preparation for having a similar API that allows multiple
exporters to exist at the same time.

Type: refactor
Signed-off-by: Paul Atkins <patkins@graphiant.com>
Change-Id: Ibd4037682742f4c2f52b4cd1346d35fb2029461d