[3587] Extended OptionalValue with equality and assignment operators.
[3587] Created CfgSubnets4 class to hold subnets configuration.
[3587] Hooked up the CfgSubnet4 class to the SrvConfig class.
[3587] Use CfgSubnets4 object to obtain information about subnets.
[3587] Added warnings to the CfgMgr::revert and SrvConfig::copy functions.
[3562] Implemented storage class for Hosts.
[3562] Basic implementation of the HostReservationParser class.
[3562] Use CfgHosts object in the SrvConfig.
[3562] HostReservationConfig parser now parses simple host reservations.

The parser works for both IPv4 and IPv6 reservations but it lacks some
validation mechanisms.
[3562] Added more tests for HostReservationParser class.
[3562] Added negative test scenarios for ipv6 host reservations.
[3587] Addressed review comments.
[3587] Fixed a couple of errors in the doxygen in CfgSubnets4.

Also, removed unused headers from cfg_subnets4.h.
[3587] Typo fix as a result of the review.
[master] Use explicit name of the first prerequisite in ext/gtest.

The $< variable is not defined in explicit rules on FreeBSD10 and causes
Kea build to fail. This change was reviewed and agreed on jabber.
[master] Merge branch 'trac3587'
[3625] Implemented the CfgSubnets6 class to hold IPv6 subnets configuration
[3625] Hooked up the CfgSubnets6 class to SrvConfig.
[3625] Use CfgSubnets6 to obtain subnet information.
[3625] Removed unused log message from libdhcpsrv.