create PktThreadPool singleton instance in dhcpsrv lib to perform start stop thread pool from hooks

we need a way to implement startPktProcessing and stopPktProcessing functions by accessing a global singleton instance which holds the PktThreadPool object in dhcpsrv lib

merge this after merging #892 (closed) - which implements most of the functionality

please review !681 (merged) and premium !p90

Edited Mar 06, 2020 by Razvan Becheriu

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