CID 352848: split xfrin_start() and remove dead code

CID 352848: split xfrin_start() and remove dead code

This commit separates TLS context creation code from xfrin_start() as
it has become too large and hard to follow into a new
function (similarly how it is done in dighost.c)

The dead code has been removed from the cleanup section of the TLS
creation code:

* there is no way 'tlsctx' can equal 'found';
* there is no way 'sess_cache' can be non-NULL in the cleanup section.

Also, it fixes a bug in the older version of the code, where TLS
client session context fetched from the cache would not get passed to
isc_nm_tlsdnsconnect().