lib: add mt-safe variants for stream_fifo ops

lib: add mt-safe variants for stream_fifo ops

stream_fifo is used as our standard internal message queue. Message
queues are useful in multithreaded environments. Up until now I have
been doing my own synchronization when using stream_fifo in this way;
this patch gets rid of the need for that boilerplate and decreases the
risk of locking mistakes when working with this datastructure.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>