optimize codec selection and format changing code

force all transcode paths to use AST_FORMAT_SLINEAR as the frames pass through the bridge (can be disabled using the 'transcode_via_sln' setting in th 'options' setting in asteris.conf)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@5376 65c4cc65-6c06-0410-ace0-fbb531ad65f3