[xfrout-thr] send error response in a separate thread.

also renamed _reply_query_with_error_rcode as _reply_with_error_rcode
simply because the former sounded awkward and redundant.
[xfrout-thr] use a separate thread to build/send xfr response.
[xfrout-thr] (refactor) rearranged ctor params for XfroutResponder

so we can omit parameters that are only used for normal responses when
the response is an error.
[xfrout-thr] cleanup: make a class attr 'private' where possible.
[xfrout-thr] move statistics and log pre/post respond() to XfroutResponder.

this is not only cleaner but also necessary now that respond() is run in a
different thread.
[xfrout-thr] catch exceptions in responding within XfroutResponder

this is necessary now this is run in a separate thread.
one unit test case was adjusted for this.
[xfrout-thr] don't bother to catch exception from XfroutResponder

as there's nothing special in the exception handler any more, and the
exception is caught by the caller.
[xfrout-thr] introduce a cleaner helper class for better resource cleaning.
[xfrout-thr] added doc about the purpose and notes on the cleaner.
[xfrout-thr] move make_blocking() inside XfroutResponder.

this would be a better place to do that since all send operations are now
done in this class.
[xfrout-thr] cleanup: don't create thread in XfroutServer.

it's now deferred later within XfroutSession.
Merge pull request #44 from bundy-dns/xfrout-thr

Xfrout: improved use of threads