BIND 10 #2207: define and implement (datasrc::memory::)ZoneUpdater class

[BIND 10 Development]

#2207: define and implement (datasrc::memory::)ZoneUpdater class
-------------------------------------+-------------------------------------
            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |  background zone loading
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 A subtask of #2201, depend on #2206.

 See the conceptual definition below.

 {{{#!cpp
 // Provide interfaces for staged zone loading, hiding the underlying
 // memory segment model.
 class isc::datasrc::memory::ZoneUpdater {
 public:
     // (Conceptually) load zone(s) content in memory.  This can take
     // long time.
     // For the local memory segment version, it creates a new ZoneData
 using
     // the zone table's local memory segment, and load the zone using
     // the given load_action.
     // For the shared memory segment version, this is probably no-op
     // (the time consuming part should have been done by the "b10-memmgr")
     virtual void load() = 0;

     // Install the new zone image so it can actually be used by the
 caller.
     // For the local memory segment version, it saves the old version of
     // ZoneData from the table and inserts the newly created one  in
 load()
     // to the zone table.
     // The shared memory version would just internally create
     // a new MemorySegment object with a given shared segment parameter,
     // and pass it to the caller via the install_action.
     virtual void install() = 0;

     // (Conceptually) release any resource used for the older version of
 zone.
     // This can take long time.
     // The local memory version destroys the saved (in install()) old
 version
     // of ZoneData.
     // The share memory version would probably be no-op.
     virtual void cleanup() = 0;
 };
 }}}

 Aand implement the `ZoneUpdaterLocal` derived class:

 {{{#!cpp
 // local memory segment version of ZoneUpdater
 class ZoneUpdaterLocal : public ZoneUpdater {
 public:
     ZoneUpdaterLocal(ztable_segment, load_action, install_action);
     ...
 };
 }}}

 Finally, add the getZoneUpdater() method to `ZoneTableSegment`
 (see #2206) and implement it for `ZoneTableSegmentLocal`.

-- 
Ticket URL: <http://bind10.isc.org/ticket/2207>
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BIND 10 Development