[rosserial_python] Stop old thread before recreating new SerialClient

[708yamaguchi]

In serial_node.py, when an "Unexpected Error" occurs, the client variable is recreated.
However, the write_thread thread used in the client variable is not released at that time.
This will increase the number of threads as "Unexpected Error" is repeated.

I checked the number of thread in serial_node.py with the following command.

import threading
threading.active_count()

If the number of threads exceeds the ulimit -u limit, no other processes can be launched.
For your reference, I share the error logs I am getting with my robot.

OpenBLAS blas_thread_init: pthread_create failed for thread 10 of 12: Resource temporarily unavailable
OpenBLAS blas_thread_init: RLIMIT_NPROC 126864 current, 126864 max

thread.error: can't start new thread

https://gist.github.com/708yamaguchi/ff32e5921570e7e004836a758665acde