Added @todos to the selectSubnet methods of CfgSubnets4 and CfgSubnets6.

This is to unify the naming with the Subnet6 tests as well as with the
name of the function under test.

The $< variable is not defined in explicit rules on FreeBSD10 and causes
Kea build to fail. This change was reviewed and agreed on jabber.

 - better handling of leases without HWAddr
 - Implement tests for Lease{4,6}::toText()

Also, removed unused headers from cfg_subnets4.h.

    Corrected two replacments of getSignalStates() that were overlooked.

 - Comments updated
 - getRawHWAddr renamed to getHWAddrVector and moved to Lease class
 - HWAddr object is now copied in copy ctor and = operator