stplib: Reduction of the number of threads

The number of threads was reduced for the simplification of processing.

There was three sending BPDU threads before.
 - send Config BPDU thread
 - send TopologyChange BPDU thread
 - send TopologyChangeNotification BPDU thread

They were unified to one thread.
 - send BPDU thread

Signed-off-by: WATANABE Fumitaka <watanabe.fumitaka@nttcom.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>