add PySparkOvertimeMonitor to avoid exceeding time budget (#923)

* merging

* clean commit

* Delete mylearner.py

This file is not needed.

* fix py4j import error

* more tolerant cancelling time

* fix problems following suggestions

* Update flaml/tune/spark/utils.py

Co-authored-by: Li Jiang <bnujli@gmail.com>

* remove redundant model

* Update test/spark/custom_mylearner.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* add docstr

* reverse change in gitignore

* Update test/spark/custom_mylearner.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

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Co-authored-by: Li Jiang <bnujli@gmail.com>
Co-authored-by: Chi Wang <wang.chi@microsoft.com>