Compilation warnings on Ubuntu 19.10/g++ 9.2.1

There are several compilation warnings:

labelsequence_unittest.cc: In member function ‘virtual void {anonymous}::ExtendableLabelSequenceTest_extendableLabelSequence_Test::TestBody()’:
labelsequence_unittest.cc:998:33: warning: implicitly-declared ‘constexpr isc::dns::LabelSequence& isc::dns::LabelSequence::operator=(const isc::dns::LabelSequence&)’ is deprecated [-Wdeprecated-copy]
  998 |     els = LabelSequence(ls1, buf);
      |                                 ^
In file included from labelsequence_unittest.cc:11:
../../../../src/lib/dns/labelsequence.h:125:5: note: because ‘isc::dns::LabelSequence’ has user-provided ‘isc::dns::LabelSequence::LabelSequence(const isc::dns::LabelSequence&)’
  125 |     LabelSequence(const LabelSequence& ls):
      |     ^~~~~~~~~~~~~

(repeated many times throughout the file.

cfg_option_unittest.cc: In member function ‘virtual void {anonymous}::CfgOptionTest_replace_Test::TestBody()’:
cfg_option_unittest.cc:297:28: warning: implicitly-declared ‘isc::dhcp::OptionDescriptor& isc::dhcp::OptionDescriptor::operator=(const isc::dhcp::OptionDescriptor&)’ is deprecated [-Wdeprecated-copy]
  297 |     desc = cfg.get("isc", 2);
      |                            ^
In file included from cfg_option_unittest.cc:17:
../../../../src/lib/dhcpsrv/cfg_option.h:104:5: note: because ‘isc::dhcp::OptionDescriptor’ has user-provided ‘isc::dhcp::OptionDescriptor::OptionDescriptor(const isc::dhcp::OptionDescriptor&)’
  104 |     OptionDescriptor(const OptionDescriptor& desc)
      |     ^~~~~~~~~~~~~~~~

This also affects client_class_def_unittest.cc, cfg_subnets4_unittest.cc, subnet_unittest.cc and config_backend_unittest.cc (and possibly more.