ipfix-export: support creating multiple exporters

ipfix-export: support creating multiple exporters

The existing api set_ipfix_exporter only allows for the creation of
a single exporter. In some cases it is desirable to export data to
multiple different destinations.  Allow users to create multiple
ipfix exporters to support this.

Add a new api that allows for the creation of multiple exporters, and
store them in a pool of exporters. The exporter created by the old API
will always be in index 0 of the pool. Exporters created by the new API
will be given the next available index in the pool, and will return this
index to the API caller so that they can track the exporter they created.
The collector_address is the key for the exporter, so changes can be made
by doing a further call to the API with the same collector_address.

Type: improvement
Signed-off-by: Paul Atkins <patkins@graphiant.com>
Change-Id: Id71c98cffcf8d141d890b40fb90a40b90a91d1d6