Fix for hang due to mp.Pool in bootstrap_stderr

[ankitgola005]

mp.pool created in bootstrap_stderr is not joined with pool.join() after it is finished, causing a hang-like situtation when too many processes get spawned.
From the documentation at https://docs.python.org/3/library/multiprocessing.html#multiprocessing.pool.Pool.join,
Pool.close() only lets pool know to stop accepting further multiprocess requests. In absence of join, the resources are not relinquished.

Starting Python >= 3.3, Pool supports CM protocol.
Replaced existing implementation with a mp.pool CM to automatically handle pool.close() and pool.join().

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