BIND 10 #2835: add interface to get properties of datasrc clients from ClientList

[BIND 10 Development]

#2835: add interface to get properties of datasrc clients from ClientList
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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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 subtask of #2830.  no dependency.

 We'll need some generic way of getting a list of data source clients
 with properties of "cache" in a `ClientList`.  That will be needed
 to decide whether we need to do "map" operation on a specific client,
 etc.

 First, we should introduce the name of client.  It would be the same
 as the data source type like "sqlite3", but can be configured.
 The names should be unique in the client list.

 Then, through an added method to `ClientList`, we'll get a
 (conceptual) list of following:
 - client name
 - cache is enabled or not
 - if cache is enabled, some cache-specific parameters. (this would
   probably have to be opaque, maybe in the form of ConstElementPtr)

 Define a set of such info and add a method to `ClientList` so the
 caller can go through this conceptual list.

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Ticket URL: <http://bind10.isc.org/ticket/2835>
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