[#2719] Adds offer_lft to globali and network scopes

[#2719] Adds offer_lft to globali and network scopes

offer-lft added to global, shared-network4, and subnet4
Can be set/fetched/parsed but does nothing yet.

src/lib/dhcpsrv/parsers/simple_parser4.cc
src/lib/dhcpsrv/cfg_globals.*
    Add offer-lft to global, shared-network4, and subnet4 values

src/lib/dhcpsrv/network.*
    Add offer_lft_ to Network4

src/lib/dhcpsrv/parsers/base_network_parser.*
    BaseNetworkParser::parseOfferLft() - new function

src/lib/dhcpsrv/parsers/dhcp_parsers.cc
    Subnet4ConfigParser::initSubnet()

src/lib/dhcpsrv/parsers/shared_network_parser.cc
    SharedNetwork4Parser::parse() - add call to parseOfferLft()

src/lib/dhcpsrv/tests/alloc_engine4_unittest.cc
    TEST_F(AllocEngine4Test, discoverOfferLft) - new test, currently
    fails because impl is missing.

src/lib/dhcpsrv/tests/cfg_shared_networks4_unittest.cc
src/lib/dhcpsrv/tests/cfg_subnets4_unittest.cc
src/lib/dhcpsrv/tests/dhcp_parsers_unittest.cc
src/lib/dhcpsrv/tests/network_unittest.cc
src/lib/dhcpsrv/tests/shared_network_parser_unittest.cc
    Updated tests to account for offer-lft

src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_parser.yy
src/bin/dhcp4/json_config_parser.cc
    Added offer-lft

src/bin/dhcp4/tests/get_config_unittest.cc
    Regenerated tests.