limits CrdsGossipPull::pull_request_time size (#15793)

There is no pruning logic on CrdsGossipPull::pull_request_time
https://github.com/solana-labs/solana/blob/79ac1997d/core/src/crds_gossip_pull.rs#L172-L174
potentially allowing this to take too much memory.

Additionally, CrdsGossipPush::last_pushed_to is pruning recent push
timestamps:
https://github.com/solana-labs/solana/blob/79ac1997d/core/src/crds_gossip_push.rs#L275-L279
instead of the older ones.

Co-authored-by: Nathan Hawkins <utsl@utsl.org>

core/src/crds.rs

@@ -264,6 +264,11 @@ impl Crds {
             .map(move |i| self.table.index(*i))
     }
 
+    /// Returns number of known pubkeys (network size).
+    pub(crate) fn num_nodes(&self) -> usize {
+        self.records.len()
+    }
+
     pub fn len(&self) -> usize {
         self.table.len()
     }