[3087] Use IOServicePtr consistently in DHCP-DDNS

Some classes were using references to isc::asiolink::IOService, others
where using d2::IOServicePtr.  The latter is now used throughout for
consistency as well as support for future, possible, multi-threaded
implementation.
[3087] Initial implementation of d2::NameAddTransaction

This interrim checkin includes the initial implementation of
NameAddTransaction to b10-dhcp-ddns.  It provides the preliminary
state model needed to add forward and/or reverse DNS entries for
and FQDN.  It does not yet construct the actual DNS update requests.
[3087] Added state handler unit tests for D2::NameAddTransaction

Created unit tests for all of the states implemented by NameAddTransaction
in b10-dhcp-ddns.  The class lacks only proper construction of DNS update
requests.