Do Proof of History verification before appending entries to the bank

Note: replicate_stage is still using `process_entries()` because
changing it to `process_blocks()` causes the `test_replicate` test to
fail.

src/lib.rs

@@ -52,6 +52,7 @@ extern crate bincode;
 extern crate byteorder;
 extern crate chrono;
 extern crate generic_array;
+extern crate itertools;
 extern crate libc;
 #[macro_use]
 extern crate log;