Add multi-server routing to exp module

[aisk]

Route keys across several servers in the exp clients.

• connect_multiple on both clients; keys are placed by jump consistent hash (Lamping & Veach) over a pluggable key hash function, so resizing the server list relocates only ~1/n of the keys
• run_batch splits operations per server (one pipelined round trip each) and returns results in input order; validation still happens before anything is written
• noop now health-checks every server; debug routes by key
• The Operation trait gained a key() accessor to support routing