BIND 10 #2984: Add remaining (15+) hooks into the DHCPv6 Server

[BIND 10 Development]

#2984: Add remaining (15+) hooks into the DHCPv6 Server
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            Reporter:  stephen       |                        Owner:
                Type:  enhancement   |  UnAssigned
            Priority:  medium        |                       Status:
           Component:  dhcp6         |  reviewing
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-DHCP-20130731
         Sub-Project:  DHCP          |                   Resolution:
Estimated Difficulty:  0             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Changes (by tomek):

 * owner:  tomek => UnAssigned
 * status:  accepted => reviewing


Comment:

 The code is ready for review.

 - Four extra hook points were implemented:
   buffer6_receive, lease6_renew, lease6_release, buffer6_send.
 - skip flag in pkt6_send changed meaning: it now means skip packing.
 - Pkt6::data_ is now public field and getData() method was removed
   (it is now safer and easier to use)

 Note that dhcp6_srv_unittest.cc has been split into several files. In
 particular hooks-related tests are in a hooks_unittest.cc. I'm somewhat
 uneasy with the content of the new dhcp6_test_utils.h header. It is almost
 exact copy of the class that used to reside in dhcp6_srv_unittest.cc. It
 has too much code for a header, but I did not want to split it further
 into cc+h without getting extra opinion. Please advise if it is ok to keep
 that much code in header or not.

 This is not as full featured set of hooks as we initially planned, but it
 does cover all the features we have now. I consider this a reasonable
 compromise between our plans and time restrictions.

 One notable missing callout is lease6_expire, but we can't implement it
 until we have housekeeping routines (see #2345). I have created ticket
 #3066 for that task.

 Please review.

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Ticket URL: <http://bind10.isc.org/ticket/2984#comment:3>
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