[kacy]

Resharding didn't work. This fixes the audit's MIGRATE bug and the orphaned-keys bug, plus three more problems found along the way.

What was wrong

• No ASKING: MIGRATE sent RESTORE straight to the target. The target doesn't own a slot it is importing, so it answered MOVED, and every MIGRATE of a key failed.
• No AUTH: MIGRATE had no way to log in to a target with a password.
• COPY ignored: COPY was parsed but the local key was deleted anyway.
• Orphaned keys: the source sent ASK only for keys that MIGRATE had moved. A key created while the slot was migrating stayed on the source, and was left behind when the slot was reassigned.
• CLI reshard: if a key failed to move, GETKEYSINSLOT listed it again forever. If the loop did end, the CLI still reassigned the slot that held the key.

What changed

• MIGRATE pipelines [AUTH] ASKING RESTORE to the target and reads every reply. It takes AUTH password and AUTH2 username password, as Redis does. It honors COPY, and replies NOKEY for a missing key, also as in Redis.

• check_slot_with_migration returns a SlotCheck enum. For a migrating slot it returns AskIfMissing, and the connection layer then asks the shards which of the command's keys still exist:

  • all of them: serve the command here;
  • none: send ASK;
  • some: send TRYAGAIN, as Redis Cluster does.

  This replaces the per-slot set of migrated keys, which grew with every key moved. The migration now keeps only a count.

• ember-cli cluster reshard passes -a to MIGRATE as AUTH, treats NOKEY as moved, and stops with an error on a key it can't move, leaving that slot with the source.

Tests
New integration tests:

• a slot moved by hand: a key moved by MIGRATE, a key still on the source, a key written during the move (ASK, then ASKING on the target), a multi-key command split across the nodes (TRYAGAIN), and SETSLOT NODE. Against main it fails at the first MIGRATE with MOVED.
• MIGRATE ... COPY keeps the local key.
• MIGRATE to a target with a password: NOAUTH without AUTH, and the key stays put; it succeeds with AUTH pw.
• ember-cli cluster reshard moves 5 slots with 10 keys and leaves none behind. Against main, this loop never ends.

The coordinator's unit tests are updated for SlotCheck.

Not covered: MIGRATE's KEYS form for several keys, which redis-cli --cluster uses. Only ember-cli reshard is supported.