add freeze_lock() and fix par_process_entries() failure to detect self conflict (#4415)

* add freeze_lock and fix par_process_entries failure to detect self conflict

* fixup

* fixup

runtime/src/bank.rs

@@ -31,7 +31,7 @@ use solana_sdk::transaction::{Result, Transaction, TransactionError};
 use std::borrow::Borrow;
 use std::cmp;
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
-use std::sync::{Arc, RwLock};
+use std::sync::{Arc, RwLock, RwLockReadGuard};
 use std::time::Instant;
 
 type BankStatusCache = StatusCache<Result<()>>;
@@ -191,6 +191,10 @@ impl Bank {
         self.slot
     }
 
+    pub fn freeze_lock(&self) -> RwLockReadGuard<Hash> {
+        self.hash.read().unwrap()
+    }
+
     pub fn hash(&self) -> Hash {
         *self.hash.read().unwrap()
     }