Using bus message to call function flush_head/feed_head in main thread
to protect private data operation.
A better fix may be to listen to bus message "CODEC_NOT_FOUND" and
"MISSING_PLUGIN" directly to avoid hooking into "unknown-type" signal.
We will revisit it in next period.
This fixes [BUGID #296]
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>