Retrieve and store the hostname only once so multiple threads do not...

Retrieve and store the hostname only once so multiple threads do not potentially initialize it at the same time.
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Merged revisions 400245 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400246 65c4cc65-6c06-0410-ace0-fbb531ad65f3