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This change provides a template "CalloutHandle store" for use in the
DHCP servers.  It replaces server-specific methods and adds the ability
to clear the stored pointers.

Packet will be sent to broadcast address if broadcast flag is set by the
client. Also, DHCPNAK is always sent to broadcast address.

mostly in documentation or in comments

reviewed by muks via jabber

includes output changes for two isc_throw.

 - DORA support added
 - REQUEST/Renewing support added
 - RELEASE support added
 - AllocationEngine now supports lease4 renewals
 - many other smaller code additions
 - many tests written

- added boundary checks for Uint32Parser
- fixed #include order
- Class renamed to Dhcp4ConfigParser
- added extra test for Uint32Parser
- Many Doxygen fixes and clean-ups

Also fix some issues from the review.

- See ControlledDhcpv4Srv class for msgq support.

- Migrated from boost::shared_array to std::vector<uint8>
- Pkt6 now have all fields private, getters/setters implemented
- Many smaller clean-ups
- Tests updated to follow new procedure
- isc::util::OutputBuffer is now used

- ChangeLog from master merged.
- dhcp4_srv cleanup (echo remnants removed, assignLease renamed)
- Pkt4::unpack() now is void and throws in case of problems

- appendRequestedOptions(), assignLease() added in dhcpv4_srv class.
- dhcpv4_srv now uses hardcoded values from 192.0.2.0/24 range.

- processDiscover(), processRequest(), copyDefaultFields(),
  appendDefaultOptions() implemented in dhcpv4_srv class

- setUint8(), setUint16(), setUint32() added to Option class.

- Pkt4 class now properly sets packet type after parsing.