Merge #14298: [REST] improve performance for JSON calls

Summary:
30973e9844 [REST] improve performance for JSON calls (Antoine Le Calvez)

Pull request description:

  JSON calls do not use the raw data generated for the .bin and .hex calls.

  By moving the raw data creation into the .bin and .hex switch branches, JSON calls' performance is improved.

  Light benchmarking indicates that fetching 2000 JSON headers is ~25% faster, fetching large JSON blocks is ~4% faster.

Tree-SHA512: 433552c89bac2469d041b48a4a991d5443e4026a3ad7dc5621685386029f22826484218642fa5130c268349a55524ecbc4e30d64c867bd6632e0edd24370cf11

Backport of Core [[bitcoin/bitcoin#14298 | PR14298]]

Test Plan:
My computer is apparently too fast to measure these perf improvements reliably, but I tried this pre- and post-patch:
```
ninja check
bitcoind -rest
time curl localhost:8332/rest/headers/2000/<some-blockhash>.json
time curl localhost:8332/rest/block/<some-blockhash>.json
```

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5783

src/rest.cpp

@@ -167,20 +167,25 @@ static bool rest_headers(Config &config, HTTPRequest *req,
         }
     }
 
-    CDataStream ssHeader(SER_NETWORK, PROTOCOL_VERSION);
-    for (const CBlockIndex *pindex : headers) {
-        ssHeader << pindex->GetBlockHeader();
-    }
-
     switch (rf) {
         case RetFormat::BINARY: {
+            CDataStream ssHeader(SER_NETWORK, PROTOCOL_VERSION);
+            for (const CBlockIndex *pindex : headers) {
+                ssHeader << pindex->GetBlockHeader();
+            }
+
             std::string binaryHeader = ssHeader.str();
             req->WriteHeader("Content-Type", "application/octet-stream");
             req->WriteReply(HTTP_OK, binaryHeader);
             return true;
         }
 
         case RetFormat::HEX: {
+            CDataStream ssHeader(SER_NETWORK, PROTOCOL_VERSION);
+            for (const CBlockIndex *pindex : headers) {
+                ssHeader << pindex->GetBlockHeader();
+            }
+
             std::string strHex =
                 HexStr(ssHeader.begin(), ssHeader.end()) + "\n";
             req->WriteHeader("Content-Type", "text/plain");
@@ -242,19 +247,21 @@ static bool rest_block(const Config &config, HTTPRequest *req,
         }
     }
 
-    CDataStream ssBlock(SER_NETWORK,
-                        PROTOCOL_VERSION | RPCSerializationFlags());
-    ssBlock << block;
-
     switch (rf) {
         case RetFormat::BINARY: {
+            CDataStream ssBlock(SER_NETWORK,
+                                PROTOCOL_VERSION | RPCSerializationFlags());
+            ssBlock << block;
             std::string binaryBlock = ssBlock.str();
             req->WriteHeader("Content-Type", "application/octet-stream");
             req->WriteReply(HTTP_OK, binaryBlock);
             return true;
         }
 
         case RetFormat::HEX: {
+            CDataStream ssBlock(SER_NETWORK,
+                                PROTOCOL_VERSION | RPCSerializationFlags());
+            ssBlock << block;
             std::string strHex = HexStr(ssBlock.begin(), ssBlock.end()) + "\n";
             req->WriteHeader("Content-Type", "text/plain");
             req->WriteReply(HTTP_OK, strHex);
@@ -391,18 +398,23 @@ static bool rest_tx(Config &config, HTTPRequest *req,
         return RESTERR(req, HTTP_NOT_FOUND, hashStr + " not found");
     }
 
-    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION | RPCSerializationFlags());
-    ssTx << tx;
-
     switch (rf) {
         case RetFormat::BINARY: {
+            CDataStream ssTx(SER_NETWORK,
+                             PROTOCOL_VERSION | RPCSerializationFlags());
+            ssTx << tx;
+
             std::string binaryTx = ssTx.str();
             req->WriteHeader("Content-Type", "application/octet-stream");
             req->WriteReply(HTTP_OK, binaryTx);
             return true;
         }
 
         case RetFormat::HEX: {
+            CDataStream ssTx(SER_NETWORK,
+                             PROTOCOL_VERSION | RPCSerializationFlags());
+            ssTx << tx;
+
             std::string strHex = HexStr(ssTx.begin(), ssTx.end()) + "\n";
             req->WriteHeader("Content-Type", "text/plain");
             req->WriteReply(HTTP_OK, strHex);