Draft: TKM 1818 pausable http client

Part of #1818 (closed)

This is experimental code for pausing/resuming HttpClient/MT-mode thread-pool threads. The MR exists strictly for allowing discussion. The code is not up to coding standards nor is it intended to be. I added a "thread-toggle" command to HA hook lib which is strictly for testing purposes and should not be considered part of the "solution".