Remove non-Bash external dependencies

[dotysan]

• sleep and date aren't part of bash
• also more portable by removing non-standard nanoseconds
• still uses clock in microseconds

Description

This is now pure bash. It might improve performance slightly by reducing syscalls to sleep and date.

Also only GNU date supports %N (nanoseconds), so it would have broken on BSD or other platforms that only support strftim(3) proper. The only drawback is internal printf %(datefmt)T supports only standard strftim(3) so we must use microseconds instead of nano.

Contributing requirements

• [x] I read the contribution guidelines in CONTRIBUTING.md.
• [x] I placed my solution in the correct solution folder.
• [x] I added a README.md with the right badge(s).
• [x] I added a Dockerfile that builds and runs my solution.
• [x] I selected drag-race as the target branch.
• [x] All code herein is licensed compatible with BSD-3.

[rbergen]

Closing this as #895 includes this PR's changes