BIND 10 #2140: Create abstract API for lease database access

[BIND 10 Development]

#2140: Create abstract API for lease database access
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                   Reporter:         |                 Owner:  UnAssigned
  stephen                            |                Status:  reviewing
                       Type:  task   |             Milestone:  Sprint-
                   Priority:         |  DHCP-20120917
  medium                             |            Resolution:
                  Component:  dhcp   |             Sensitive:  0
                   Keywords:         |           Sub-Project:  DHCP
            Defect Severity:  N/A    |  Estimated Difficulty:  0
Feature Depending on Ticket:         |           Total Hours:  0
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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Changes (by tomek):

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


Comment:

 Abstract API for lease database implemented. Please note that:

 - host reservations are not planned for 2012, so they are not implemented
 (appropriate @todo is added to the code, though)
 - backend versioning is currently simplified. Full featured versioning is
 not needed in 2012 timeframe, so it was skipped. There's a comment about
 planned version checks in LeaseMgr::getVersion() method.

 LeaseMgr handles only information that will be kept in the database.
 Therefore there are no subnet or pool information methods. Those will be
 implemented in upcoming CfgMgr (that will hold all configuration options).

 The code is available on trac2140 branch. Please review.

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Ticket URL: <http://bind10.isc.org/ticket/2140#comment:2>
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