track double spending in the txpool

Transactions in the txpool are marked when another transaction
is seen double spending one or more of its inputs.
This is then exposed wherever appropriate.

Note that being marked with this "double spend seen" flag does
NOT mean this transaction IS a double spend and will never be
mined: it just means that the network has seen at least another
transaction spending at least one of the same inputs, so care
should be taken to wait for a few confirmations before acting
upon that transaction (ie, mostly of use for merchants wanting
to accept unconfirmed transactions).

src/rpc/message_data_structs.h

@@ -95,6 +95,7 @@ namespace rpc
     uint64_t last_relayed_time;
     bool relayed;
     bool do_not_relay;
+    bool double_spend_seen;
   };
 
   typedef std::unordered_map<crypto::key_image, std::vector<crypto::hash> > key_images_with_tx_hashes;