Clean up some SIP registry-related memory leaks

1) Be sure and free at unload the epa_backend we allocate at startup
2) Do the same sip_registry cleanup at unload we do at reload

Review: https://reviewboard.asterisk.org/r/1689/
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Merged revisions 352514 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 352515 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@352519 65c4cc65-6c06-0410-ace0-fbb531ad65f3