mempool: add double spend events.

lib/mempool/mempool.js

@@ -823,6 +823,7 @@ Mempool.prototype.insertTX = co(function* insertTX(tx) {
   // not double-spending an output in the
   // mempool.
   if (this.isDoubleSpend(tx)) {
+    this.emit('conflict', tx);
     throw new VerifyError(tx,
       'duplicate',
       'bad-txns-inputs-spent',
@@ -1895,6 +1896,8 @@ Mempool.prototype.removeDoubleSpends = function removeDoubleSpends(tx) {
       spent.tx.rhash());
 
     this.removeEntry(spent, true);
+
+    this.emit('double spend', spent);
   }
 };