Make accountant thread-safe

Before this change, parallel transaction processing required locking
the full accountant. Since we only call one method,
process_verified_transaction, the global lock equates to doing no
parallelization at all.  With this change, we only lock the data that's
being written to.

src/bin/testnode.rs

@@ -32,7 +32,7 @@ fn main() {
         None
     };
 
-    let mut acc = Accountant::new_from_deposit(&deposit.unwrap());
+    let acc = Accountant::new_from_deposit(&deposit.unwrap());
 
     let mut last_id = entry1.id;
     for entry in entries {