Cassandra execute timeout within multiprocessing

[ZhiruiFeng]

For pipeline test, the sink_test.py could not work because the DBSinker could not execute the insert function to Cassandra. However, the unit test of that insert function (DBWriter.insert_new_to_frame) could work smoothly. It seems like we could not get responses from Cassandra when using multiprocessing.

Currently, I use other methods to realize the same function, but it's a very wired error.