Merge #14984: rpc: Speedup getrawmempool when verbose=true

2d5cf4c rpc: Speedup getrawmempool when verbose=true (João Barbosa)

Pull request description:

  Instead of calling `pushKV(hash, info)`, which incurs in duplicate key checks, call `__pushKV` which (currently) doesn't.

  Improves RPC `getrawmempool` and REST `/rest/mempool/contents.json`.

  Fixes #14765.

ACKs for commit 2d5cf4:

Tree-SHA512: c3e91371bb41f39e79dcef820815e1dc27fb689ca3c4bf3a00467d2215b3baecd44d9792f7a481577a5b7ae1fc6cbaa07b1cd62123b845082eba65b35c2b3ca5

Author: MarcoFalke

src/rpc/blockchain.cpp

@@ -486,7 +486,10 @@ UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose)
             const uint256& hash = e.GetTx().GetHash();
             UniValue info(UniValue::VOBJ);
             entryToJSON(pool, info, e);
-            o.pushKV(hash.ToString(), info);
+            // Mempool has unique entries so there is no advantage in using
+            // UniValue::pushKV, which checks if the key already exists in O(N).
+            // UniValue::__pushKV is used instead which currently is O(1).
+            o.__pushKV(hash.ToString(), info);
         }
         return o;
     } else {