Fix timing source dependency issues with MOH

Prior to this patch, res_musiconhold existed at the same module priority level
as the timing sources that it depends on.  This would cause a problem when
music on hold was reloaded, as the timing source could be changed after
res_musiconhold was processed.  This patch adds a new module priority level,
AST_MODPRI_TIMING, that the various timing modules are now loaded at.  This
now occurs before loading other resource modules, such that the timing source
is guaranteed to be set prior to resolving the timing source dependencies.

(closes issue ASTERISK-17474)
Reporter: Luke H
Tested by: Luke H, Vladimir Mikhelson, zzsurf, Wes Van Tlghem, elguero, Thomas Arimont
Patches:
 asterisk-17474-dahdi_timing-infinite-wait-fix_v3_branch-1.8.diff uploaded by elguero (License #5026)
 asterisk-17474-dahdi_timing-infinite-wait-fix_v3_branch-10.diff uploaded by elguero (License #5026)
 asterisk-17474-dahdi_timing-infinite-wait-fix_v3.diff uploaded by elguero (License #5026)

Review: https://reviewboard.asterisk.org/r/1578/
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Merged revisions 349194 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 349195 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@349196 65c4cc65-6c06-0410-ace0-fbb531ad65f3