Merge pull request #989 in FS/freeswitch from ~DRAGOS_OANCEA/freeswitch-dragos:opus-improve-adjustable-bitrate to master

* commit 'f47bbece':
  FS-9601: mod_opus:  make adjustable bitrate mutually exclusive with FEC enforcing on the decreasing trend, add step calculation for bitrate adjustment, fix bug on context settings