Replace `DashMap` with `papaya` for `shards_table` in server shard

The shards_table (`DashMap`) is the shared lookup table that maps partition namespaces to their owning shard. It is queried on every message routing decision (find_shard, resolve) but only written to during partition creation, deletion, or rebalancing, which is ready-heavy workload.

`papaya` is a lock-free concurrent hash map specifically optimized for read-heavy access patterns. Switching `shards_table` from DashMap to papaya should reduce contention on the hot routing path.