[0.6.2] Split /simulate calls and minor fixes

Changes

• Rather than doing 1 /simulate call, do 6 /simulate/save calls (one for each save) in parallel, and then combine them.
  • This should allow each call to be handled by a different express worker, speeding up the total request time

Architecture

• Set the number of express workers to the CPU count (no arbitrary limit)
• Add a function that creates a new worker if it detects one has died

Fixes

• Fix damage simulation taking average roll instead of rolling