core, core/types: readd transactions after chain re-org

Added a `Difference` method to `types.Transactions` which sets the
receiver to the difference of a to b (NOTE: not a **and** b).

Transaction pool subscribes to RemovedTransactionEvent adding back to
those potential missing from the chain.

When a chain re-org occurs remove any transactions that were removed
from the canonical chain during the re-org as well as the receipts that
were generated in the process.

Closes #1746

core/transaction_pool_test.go

@@ -238,3 +238,15 @@ func TestNonceRecovery(t *testing.T) {
 		t.Errorf("expected nonce to be %d, got %d", n+1, fn)
 	}
 }
+
+func TestRemovedTxEvent(t *testing.T) {
+	pool, key := setupTxPool()
+	tx := transaction(0, big.NewInt(1000000), key)
+	from, _ := tx.From()
+	pool.currentState().AddBalance(from, big.NewInt(1000000000000))
+	pool.eventMux.Post(RemovedTransactionEvent{types.Transactions{tx}})
+	pool.eventMux.Post(ChainHeadEvent{nil})
+	if len(pool.pending) != 1 {
+		t.Error("expected 1 pending tx, got", len(pool.pending))
+	}
+}