stplib: Reduction of the number of threads
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 <6c314859a5392f5f0c2f5e010c7240be8063009a@nttcom.co.jp>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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