chan_local: Switch from using a random 4 digit hex identifier to unique id

Changes chan_local channels to use an 8 digit hex identifier generated
atomically and sequentially in order to eliminate the chance of having
multiple channels with the same name during high call volume situations.

(issue ASTERISK-20318)
Reported by: Dan Cropp
Review: https://reviewboard.asterisk.org/r/2104/
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Merged revisions 372902 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 372916 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 372917 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372918 65c4cc65-6c06-0410-ace0-fbb531ad65f3