redis-trib: load info about importing/migrating slots from node.
redis-trib: initial support to fix "open" slots.

Open slots are slots found in importing or migrating slot when a
cluster check is performed.
Cluster: move slotToKeyFlush() to emptyDb().

This way we are sure to destroy the slot->key map every time we destroy
the DB, for instance when reloading a DB due to replication.
redis-trib: fix conditional otherwise always true.