Decrease overhead take 2

[joao-paulo-parity]

• Run the processes with priority
• Set stable as the default toolchain
• Improve run script
• Add more documentation
• Introduce DEBUG mode for figuring out issues with less friction.

After this PR lands:

• We should be seeing a decrease in the benchmark timings accross the board since processes are ran with priority. Examples:
  • https://github.com/paritytech/substrate/pull/9669/commits/3e8d819cd89efa24837584366f258f129c884e80
  • https://github.com/paritytech/substrate/pull/9507/commits/8da3202ab865f5d47f0f5b3e02af92a738ffc046
  • https://github.com/paritytech/substrate/pull/9675/commits/cb643f71961a62a37095d54f4afdc4162a6a199b
• The initial message will inform the active toolchain used for the benchmark.

closes #50