scheduler: Remove hashtab usage.

This is a first stab at tweaking the performance profile of the scheduler. Removing
the hashtab usage removes an extra memory allocation when scheduling something and
makes it so rescheduling does not incur any memory allocation at all.

Review: https://reviewboard.asterisk.org/r/3199/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@407958 f38db490-d61c-443f-a65b-d21fe96a405b