[recovery-netstack] IMPROVEMENTS.md: Add improvements

- Add dedicated "Performance Issues" section
- Add "Single-threaded execution" sub-section
- Add "`debug_err!` macro" sub-section
- Add "Pervasive use of `HashMap`s" sub-section
- Expand "Devices in `HashMap`s" sub-section

Change-Id: I6721b556cf21edf03f24fc21b3282b4e5fb64461