vmhost process/thread management

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The vmhost needs structures and code for handling edge-receptor threads, as well as a thread pool to allocate to handling receptors process queues.

This code needs to be called from the main thread that comes from the accumulator boot-up sequence

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basics in place in: c93111b

Still todo:

• thread pool for multi-threaded reduction of receptor processes instead of single polling by vmhost thread
• using pthread condition variables to remove polling.
• use case: communication between clock receptor and std-out edge receptor (command line shell receptor create shell/command line receptor #25)
• sending signals between receptors as part of the processing loop.

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Here's a good candidate for a thread pool library. Very simple and it looks like it's broadly used and tested. https://github.com/Pithikos/C-Thread-Pool