[REST] Handle UTXO retrieval when ignoring the mempool

Current REST API always returns empty UTXO when invoked without `/checkmempool/` URL part.

After the fix:
```
$ curl -s http://localhost:8332/rest/getutxos/0e3e2357e806b6cdb1f70b54c3a3a17b6714ee1f0e68bebb44a74b1efd512098-0.json | jq
{
  "chainHeight": 514109,
  "chaintipHash": "0000000000000000001fe76d1445e8a6432fd2de04261dc9c5915311dc7ad6de",
  "bitmap": "1",
  "utxos": [
    {
      "height": 1,
      "value": 50,
      "scriptPubKey": {
        "asm": "0496b538e853519c726a2c91e61ec11600ae1390813a627c66fb8be7947be63c52da7589379515d4e0a604f8141781e62294721166bf621e73a82cbf2342c858ee OP_CHECKSIG",
        "hex": "410496b538e853519c726a2c91e61ec11600ae1390813a627c66fb8be7947be63c52da7589379515d4e0a604f8141781e62294721166bf621e73a82cbf2342c858eeac",
        "reqSigs": 1,
        "type": "pubkey",
        "addresses": [
          "12c6DSiU4Rq3P4ZxziKxzrL5LmMBrzjrJX"
        ]
      }
    }
  ]
}
```

Before the fix:
```
$ curl -s http://localhost:8332/rest/getutxos/0e3e2357e806b6cdb1f70b54c3a3a17b6714ee1f0e68bebb44a74b1efd512098-0.json | jq
{
  "chainHeight": 514109,
  "chaintipHash": "0000000000000000001fe76d1445e8a6432fd2de04261dc9c5915311dc7ad6de",
  "bitmap": "0",
  "utxos": []
}
```

1 files changed, 20 insertions, 18 deletions

diff --git a/src/rest.cpp b/src/rest.cpp
index f47b40343b..6501dab6bf 100644
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -487,26 +487,28 @@ static bool rest_getutxos(HTTPRequest* req, const std::string& strURIPart)
     std::vector<bool> hits;
     bitmap.resize((vOutPoints.size() + 7) / 8);
     {
-        LOCK2(cs_main, mempool.cs);
-
-        CCoinsView viewDummy;
-        CCoinsViewCache view(&viewDummy);
-
-        CCoinsViewCache& viewChain = *pcoinsTip;
-        CCoinsViewMemPool viewMempool(&viewChain, mempool);
-
-        if (fCheckMemPool)
-            view.SetBackend(viewMempool); // switch cache backend to db+mempool in case user likes to query mempool
-
-        for (size_t i = 0; i < vOutPoints.size(); i++) {
-            bool hit = false;
-            Coin coin;
-            if (view.GetCoin(vOutPoints[i], coin) && !mempool.isSpent(vOutPoints[i])) {
-                hit = true;
-                outs.emplace_back(std::move(coin));
+        auto process_utxos = [&vOutPoints, &outs, &hits](const CCoinsView& view, const CTxMemPool& mempool) {
+            for (const COutPoint& vOutPoint : vOutPoints) {
+                Coin coin;
+                bool hit = !mempool.isSpent(vOutPoint) && view.GetCoin(vOutPoint, coin);
+                hits.push_back(hit);
+                if (hit) outs.emplace_back(std::move(coin));
             }
+        };
+
+        if (fCheckMemPool) {
+            // use db+mempool as cache backend in case user likes to query mempool
+            LOCK2(cs_main, mempool.cs);
+            CCoinsViewCache& viewChain = *pcoinsTip;
+            CCoinsViewMemPool viewMempool(&viewChain, mempool);
+            process_utxos(viewMempool, mempool);
+        } else {
+            LOCK(cs_main);  // no need to lock mempool!
+            process_utxos(*pcoinsTip, CTxMemPool());
+        }
 
-            hits.push_back(hit);
+        for (size_t i = 0; i < hits.size(); ++i) {
+            const bool hit = hits[i];
             bitmapStringRepresentation.append(hit ? "1" : "0"); // form a binary string representation (human-readable for json output)
             bitmap[i / 8] |= ((uint8_t)hit) << (i % 8);
         }