rpc: move UniValue in blockToJSON

Without explicitly declaring the move, these UniValues get copied,
causing increased memory usage. Fix this by explicitly moving the
UniValue objects.

Used by `rest_block` and `getblock` RPC.

src/rpc/blockchain.cpp

@@ -188,12 +188,12 @@ UniValue blockToJSON(BlockManager& blockman, const CBlock& block, const CBlockIn
                 const CTxUndo* txundo = (have_undo && i > 0) ? &blockUndo.vtxundo.at(i - 1) : nullptr;
                 UniValue objTx(UniValue::VOBJ);
                 TxToUniv(*tx, /*block_hash=*/uint256(), /*entry=*/objTx, /*include_hex=*/true, txundo, verbosity);
-                txs.push_back(objTx);
+                txs.push_back(std::move(objTx));
             }
             break;
     }
 
-    result.pushKV("tx", txs);
+    result.pushKV("tx", std::move(txs));
 
     return result;
 }