scans crds table in parallel for finding old labels (#13073)

From runtime profiles, the majority time of ClusterInfo::handle_purge
https://github.com/solana-labs/solana/blob/0776fa05c/core/src/cluster_info.rs#L1605-L1626
is spent scanning crds table finding old labels:
https://github.com/solana-labs/solana/blob/0776fa05c/core/src/crds.rs#L175-L197

This can be done in parallel given that gossip thread-pool:
https://github.com/solana-labs/solana/blob/0776fa05c/core/src/cluster_info.rs#L1637-L1641
is idle when handle_purge is invoked:
https://github.com/solana-labs/solana/blob/0776fa05c/core/src/cluster_info.rs#L1681

core/benches/crds.rs

@@ -0,0 +1,31 @@
+#![feature(test)]
+
+extern crate test;
+
+use rand::{thread_rng, Rng};
+use rayon::ThreadPoolBuilder;
+use solana_core::crds::Crds;
+use solana_core::crds_gossip_pull::CRDS_GOSSIP_PULL_CRDS_TIMEOUT_MS;
+use solana_core::crds_value::CrdsValue;
+use solana_sdk::pubkey::Pubkey;
+use std::collections::HashMap;
+use test::Bencher;
+
+#[bench]
+fn bench_find_old_labels(bencher: &mut Bencher) {
+    let thread_pool = ThreadPoolBuilder::new().build().unwrap();
+    let mut rng = thread_rng();
+    let mut crds = Crds::default();
+    let now = CRDS_GOSSIP_PULL_CRDS_TIMEOUT_MS + CRDS_GOSSIP_PULL_CRDS_TIMEOUT_MS / 1000;
+    std::iter::repeat_with(|| (CrdsValue::new_rand(&mut rng), rng.gen_range(0, now)))
+        .take(50_000)
+        .for_each(|(v, ts)| assert!(crds.insert(v, ts).is_ok()));
+    let mut timeouts = HashMap::new();
+    timeouts.insert(Pubkey::default(), CRDS_GOSSIP_PULL_CRDS_TIMEOUT_MS);
+    bencher.iter(|| {
+        let out = crds.find_old_labels(&thread_pool, now, &timeouts);
+        assert!(out.len() > 10);
+        assert!(out.len() < 250);
+        out
+    });
+}