Make the default fallback weight in Tor 10.0
This is a follow-up to #24679 (moved).
- update the default weight in parse_dir_fallback_line() to 10.0
- update the man page to reflect the new default
10.0 gives us:
- 0.5% clients bootstrapping off an authority when all fallbacks are up, * 2% when 25% are down (our replacement threshold)
- 7% when 40% are down (our worst case scenario)