Zebra should tell users about disk space requirements

[mpguerra]

Is your feature request related to a problem? Please describe.

During our first alpha user studies (#1501) we discovered that users may be concerned about disk space requirements of a running Zebra node.

Describe the solution you'd like

Even though we list the disk space requirements on our README, we could be more explicit about:

1. Telling users about disk space requirements on zebrad start
   • our current tracing logs can't really provide this information, because the log messages scroll past really fast
2. Adding more helpful hints on panics due to running out of space

Describe alternatives you've considered

We already have this info in our README and are restructuring it to make this information more prominent in #1617

[teor2345]

I reworded this ticket, because "warn" has a very specific meaning in Zebra. "Warn" is a tracing a log level, and logging a warning message doesn't really do what we want here.

[teor2345]

This will probably get done as part of the README cleanup, or we can re-open it if it becomes an issue again.