Make SCOPED_LOCK use RAII_VAR.

This fixes an issue with requiring SCOPED_LOCK to be the last variable
declaration and removes duplicate code in the process.

Review: https://reviewboard.asterisk.org/r/2665/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393919 65c4cc65-6c06-0410-ace0-fbb531ad65f3