BIND 10 #2832: extend data source config to specify shmem segment params

[BIND 10 Development]

#2832: extend data source config to specify shmem segment params
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                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:  task   |             Milestone:  Next-Sprint-
                   Priority:         |  Proposed
  medium                             |              Keywords:
                  Component:         |             Sensitive:  0
  Unclassified                       |           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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 subtask of #2830.  no dependency.

 We'll extend the data source configuration so we can specify the type
 of cache (in memory) memory segment type and its parameter, if any.
 It would look like this:

 {{{
 "IN": [{
     "type": "sqlite3",
     "params": {"database_file":
 "/Users/jinmei/src/isc/git/bind10/zone.sqlite3"},
     "cache-zones": ["jinmei.org", "kame.net"],
     "cache-enable": true,
     "cache-segment-type": "mmap",
     "cache-segment-params": {
       "mapped-file-base": "/usr/local/var/bind10/zone.mapped"
     }
 }]
 }}}

 cache-segment-type has the default of "local".
 the default of cache-segment-params is empty.

 For now we only extend the spec.  It will be simply ignored if
 specified in the actual config.

 Note: mapped-file-base would have some reasonable default so the user
 can omit it.

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Ticket URL: <http://bind10.isc.org/ticket/2832>
BIND 10 Development <http://bind10.isc.org>
BIND 10 Development