Txn conflicts

This builds on #3656 and #3669.

It adds a "walletconflicts" array to wallet transaction JSON output, identifying transactions that share the same normalized transaction ID (that have exactly the same inputs and outputs).

I need to sleep on the last commit, I'm afraid it might open up services to a double-spend attack if a blockchain re-org is done.

Example output from the txnmall.sh regression test, spending 1 BTC from account "foo", then spending 2 BTC of the change from that transaction and spending it from account "bar".  After a mutant of the "foo" transaction is then mined listtransactions reports:

```
    {
        "account" : "foo",
        "address" : "mzkQgu25PwDiizVe386isePJtcvcuUfe92",
        "category" : "conflicted",
        "amount" : -1.00000000,
        "fee" : 0.00000000,
        "confirmations" : -1,
        "txid" : "37ae388f1d2a4df39a1eae251bf8ee43000691284d6a28077154b9648ec66318",
        "normtxid" : "fe7355741ed22c056469e22797f15cc31916e0026828c6daf3110ff76bb81d1e",
        "walletconflicts" : [
            "9c5f1b72f18c28b7e0e501c7125329333d0baeb4def08c565a1957031b9d3bbb"
        ],
        "time" : 1392336916,
        "timereceived" : 1392336916
    },
    {
        "account" : "foo",
        "address" : "mzkQgu25PwDiizVe386isePJtcvcuUfe92",
        "category" : "send",
        "amount" : -1.00000000,
        "fee" : 0.00000000,
        "confirmations" : 1,
        "blockhash" : "0000b1e01161164fe2622491b5748ecfcd69a4305842815f17d0f1c4888d6dc6",
        "blockindex" : 1,
        "blocktime" : 1392336917,
        "txid" : "9c5f1b72f18c28b7e0e501c7125329333d0baeb4def08c565a1957031b9d3bbb",
        "normtxid" : "fe7355741ed22c056469e22797f15cc31916e0026828c6daf3110ff76bb81d1e",
        "walletconflicts" : [
            "37ae388f1d2a4df39a1eae251bf8ee43000691284d6a28077154b9648ec66318"
        ],
        "time" : 1392336916,
        "timereceived" : 1392336917
    },
    {
        "account" : "bar",
        "address" : "mzkQgu25PwDiizVe386isePJtcvcuUfe92",
        "category" : "conflicted",
        "amount" : -2.00000000,
        "fee" : 0.00000000,
        "confirmations" : -1,
        "txid" : "a41295cf488bb3baa654522658e6847cfd385737f82a3447dc1cddb77d7dfd7c",
        "normtxid" : "bfcb414d3a3372fb22008e56a530585f4d50644de5c6fc0c6ad2c0382f9c945b",
        "walletconflicts" : [
        ],
        "time" : 1392336917,
        "timereceived" : 1392336917
    }
```