[Rework] Replace broken Jump Hash with Ring Hash (Ketama) for consistent upstream...

[Rework] Replace broken Jump Hash with Ring Hash (Ketama) for consistent upstream hashing

Jump Consistent Hash (Lamping & Veach 2014) only handles bucket
addition/removal at the end of the range.  When an upstream in the
middle failed, the old code rehashed with mum_hash_step and retried
up to 20 times, which destroyed the consistency property: keys that
mapped to the dead node were redistributed randomly instead of
deterministically, and didn't return when the node recovered.

Replace with a Ketama-style ring hash:
- Each alive upstream gets MAX(weight,1)*100 virtual nodes on a
  sorted hash ring (keyed by name, order-independent).
- Lookup is a binary search: O(log(n*v)) instead of O(ln n) * retries.
- When an upstream fails, only its ~1/n fraction of keys slide to the
  next ring point — true minimal disruption.
- When it recovers, the same keys return — true consistency.
- The 'except' parameter walks forward on the ring instead of rehashing.
- Ring is rebuilt lazily (dirty flag set on active/inactive transitions).