BIND 10 #1879: unify "scoped socket" helper class

[BIND 10 Development]

#1879: unify "scoped socket" helper class
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            Reporter:  jinmei        |                        Owner:
                Type:  defect        |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  Unclassified  |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  Core          |              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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 We have several copies of something like `ScopedSocket` (see, e.g.,
 src/lib/util/io/socketsession.cc).  To avoid having duplicate code
 we should unify them.  maybe it makes most sense to be moved from
 inside socketsession.cc to a separate header file under util/io, and
 have all others refer to it.

 See also http://bind10.isc.org/ticket/1820#comment:8

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Ticket URL: <http://bind10.isc.org/ticket/1879>
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