- SimpleParser concept implemented
 - Converted 4 parsers (option data, option data list, option defintion,
   option definition list)
 - updated unit-tests
 - converted other parsers (HostReservationParser{4,6}, ClientClassDefParser)
   to use those new parsers
 - converted kea-dhcp{4,6} to use those new parsers

Conflicts:
	src/bin/dhcp6/json_config_parser.cc
	src/lib/dhcpsrv/parsers/dhcp_parsers.cc

 It now contains vector of ElementPtr, rather than ConstElementPtr

# Conflicts:
#	src/lib/dhcpsrv/pool.h

src/bin/dhcp6/dhcp6_srv.h
src/bin/dhcp6/dhcp6_srv.cc
    Removed unused ctx parameter from Dhcpv6Srv::appendRequestedOptions()

Trivial changes.