ofproto-dpif-ipfix: add Exporting Process Reliability Statistics

This patch adds support for The Exporting Process Reliability Statistics
Option Template, which is defined in RFC 5101, Section 4.3. For now, this
Option Template includes only exporting process ID scope field and a
single counter - notSentPacketTotalCount.

Currently, Data Records that contain exporting process statistics are
transmitted at regular intervals, immediately after sending (Options) Template
Records. This behavior could be changed later on to allow the interval of
sending exporting process statistics to be configurable as RFC recommends it.
According to RFC 5101, Section 10.3.6. the frequency of the (Options) Template
transmissions MUST be configurable. OvS doesn't comply to this requirement.
Therefore, it seems to be a good idea to make both intervals configurable at
the same time.

Signed-off-by: Przemyslaw Szczerbik <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>