BIND 10 #2913: update CacheConfig and ZoneWriter using class version of ZoneDataLoader

[BIND 10 Development]

#2913: update CacheConfig and ZoneWriter using class version of ZoneDataLoader
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                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:  task   |             Milestone:  Next-Sprint-
                   Priority:         |  Proposed
  medium                             |              Keywords:
                  Component:  data   |             Sensitive:  0
  source                             |           Sub-Project:  DNS
               CVSS Scoring:         |  Estimated Difficulty:  0
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
  shared memory data source          |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 This is a followup of #2912 and depends on it (also depends on
 #2834 but assuming it's merged by the time we work on this task).

 We'll update the `CacheConfig` class as follows:
 {{{#!cpp
 class CacheConfig {
 public:
     // A factory to create zone data loader with the context of cache
 config.
     typedef boost::function<ZoneDataLoader*(MemorySegment&, const
 RRClass&,
                                             const Name&, ZoneData*)
         ZoneDataLoaderFactory;

     // this will replace getLoadAction.  cache_client and options won't be
     // used for now.  cache_client will be the corresponding
 InMemoryClient
     // and options are some numeric type that supports operator| (logical
 or).
     // it mostly works like getLoadAction(), but returns a factory functor
     // (defined above) depending on the cache configuration.
     ZoneDataLoaderFactory createZoneDataLoader(
         const RRClass& rrclass, const Name& zname,
         DataSourceClientPtr cache_client, Options options);
 }
 }}}

 We'll also update `ZoneWriter` (it's not a base class after #2850) so
 - its constructor takes `ZoneDataLoaderFactory`
 - load and install methods will be revised as follows:
 {{{#!cpp
 void
 ZoneWriter::load() {
     // loader_ is of scoped_ptr<ZoneDataLoader>
     loader_.reset(loader_factory_(segment_->getMemorySegment(), zclass,
 zname,
                                   segment_->getHeader().getZoneTable()));
     loader_->load();
 }

 void
 ZoneWriter::install() {
     loader_->commit();          // note that it's no-op for now
     table->addZone();
 }
 }}}

 When this is done, we could remove the function versions of
 `loadZoneData()` (but we could keep them if we find it convenient).
 We can also remove `CacheConfig::getLoadAction()` and load_action.h.

 Also, if the work so far is not big, consider adding
 `loadIncrement(count)` to ZoneWriter.  That should be basically trivial.

-- 
Ticket URL: <http://bind10.isc.org/ticket/2913>
BIND 10 Development <http://bind10.isc.org>
BIND 10 Development