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

The reference could be removed because the TEST_DATA_DIR was unused in the
asiodns unit tests. In fact the folder it pointed to did not exist. Removal
of the call to addDataPath allowed for not including and compiling the
dns/tests/unittest_util.h and dns/tests/unittest_util.cc.

The major reason for this change was to remove the dependency on the files
located in the dns/tests because due to the bug in automake they could not
be included in build using an absolute path specified with $(top_srcdir).

Note that when the AUTOMAKE_OPTIONS = subdir-objects is specified, the
object files are created in the subdirectories. When the location of the
source file is specified using a variable like $(top_srcdir) the name of
the variable is treated as a name of the subdirectory, not a path to the
file located out of the tree. Hence, we may need to avoid including files
like this when subdir-objects is in force.

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

 - 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

The parser works for both IPv4 and IPv6 reservations but it lacks some
validation mechanisms.