Changes:
 - hwaddr.h - added MAX_HWADDR_LEN
 - hwaddr.cc - modified constructor to throw InvalidParameter
            if input address is too big
 - hwaddr_unittest.cc - added tests for new constructor throws

 - lease_mgr.h - removed HWADDR_MAX
 - mysql_lease_mgr.cc - added logic to set SQL mode to STRICT
 - tests/mysql_lease_mgr_unittest.cc - modified tests that were
    failing on STRICT mode, to expect exceptions.