Resolve Stack Overflow

[ec2]

Summary of changes
Changes introduced in this pull request:

• Adds a custom global Rayon threadpool so we can easily identify where panics or stack overflows happen in the future for tasks spawned in the global Rayon threadpool
• We have a minimum on the length that Rayon will split when doing parallel iteration, so that Rayon wont split into too many chunks, which is what was causing the stack overflow.

Reference issue to close (if applicable)

Closes
#869

Other information and links

[CLAassistant]

All committers have signed the CLA.

[ghost]

Nice and trim!

[cryptoquick]

Nice! I prefer once_cell, but I suppose it's fine if we use both.