NIFI-12693: Moved notification of python process that a Processor was…

[markap14]

… removed to a background (virtual) thread. Also noted in testing that in one instance a Python Processor never became

valid because it had cached property descriptors before the processor was fully initialized, so updated code to ensure that we do not cache values before initialization is completed.

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NIFI-00000

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[exceptionfactory]

Thanks @markap14, the adjustments to the relationship handling behavior look good. +1 merging