move last_id age checking into the HashMap

* allows for simpler chaining of banks
  * looks 1.5-2% faster than looping through a VecDequeue

TODO: remove timestamp()?

benches/bank.rs

@@ -5,6 +5,7 @@ extern crate solana;
 extern crate test;
 
 use solana::bank::*;
+use solana::hash::hash;
 use solana::mint::Mint;
 use solana::signature::{Keypair, KeypairUtil};
 use solana::system_transaction::SystemTransaction;
@@ -40,6 +41,13 @@ fn bench_process_transaction(bencher: &mut Bencher) {
             tx
         }).collect();
 
+    let mut id = bank.last_id();
+
+    for _ in 0..(MAX_ENTRY_IDS - 1) {
+        bank.register_entry_id(&id);
+        id = hash(&id.as_ref())
+    }
+
     bencher.iter(|| {
         // Since benchmarker runs this multiple times, we need to clear the signatures.
         bank.clear_signatures();