v4 Parsing now allows renew/rebind timers to be unspecified,
and kea-dhcp4 logic now matchs ISC DHCP:

1. Send T2 only if it is specified and is less than lease lifetime
2. Send T1 only if it is specified and is less than either T2
if specified, or lease lifetime in the absence of T2

doc/guide/dhcp4-srv.xml
    Updated discussion of rebind/renew-timers

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::assignLease() - modified to sanity check
    T1 and T2

src/bin/dhcp4/tests/config_parser_unittest.cc
src/bin/dhcp4/tests/get_config_unittest.cc
src/bin/dhcp4/tests/simple_parser4_unittest.cc
src/bin/dhcp6/tests/get_config_unittest.cc
src/lib/dhcpsrv/tests/cfg_shared_networks4_unittest.cc
src/lib/dhcpsrv/tests/cfg_shared_networks6_unittest.cc
    updated numerous tests

src/lib/dhcpsrv/network.cc
    Network::toElement() - only outputs T1, T2, and
    valid lifetime if they are specified

src/lib/dhcpsrv/network.h
    Network() - inits t1_, t2_, and valid_ to unspecified
    Triplet value

src/lib/dhcpsrv/parsers/dhcp_parsers.cc
    Subnet4ConfigParser::initSubnet() - allow renew-timer and
    rebind-timer to be optional

src/lib/dhcpsrv/parsers/simple_parser4.cc
    SimpleDefaults SimpleParser4::GLOBAL4_DEFAULTS - removed
    entries for renew-timer and rebind-timer