[master] DHCPv4 renew/rebind-timers are now optional

    Merge branch 'trac5596'

diff --cc src/bin/dhcp4/tests/config_parser_unittest.cc
Simple merge

diff --cc src/lib/dhcpsrv/network.cc
Simple merge

diff --cc src/lib/dhcpsrv/parsers/simple_parser4.cc
Simple merge