MoH: Don't restart stream on repeated start calls

Currently, music on hold will stop and then start again from the
beginning if ast_moh_start() is called multiple times. This can happen
if a call is put on hold repeatedly (the channel receives multiple
HOLD control frames) and can be triggered from ARI by starting MoH on a
channel multiple times. This is fairly jarring/annoying to users.

This change prevents MoH from being restarted if the requested music
class is the same as the one currently playing.

Review: https://reviewboard.asterisk.org/r/3615/
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Merged revisions 416150 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 416151 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 416152 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@416153 65c4cc65-6c06-0410-ace0-fbb531ad65f3