Try fixing the performance issues of the "vertx-web-kotlinx" portion …

…in the "single query" and "JSON serialization" tests in the [Continuous Benchmarking results](https://tfb-status.techempower.com/) by using static kotlinx.serialization serializers

The "vertx-web-kotlinx" results in the Continuous Benchmarking results are much lower than those of the "vertx-web-kotlin-coroutines" portion. See [the latest results](https://www.techempower.com/benchmarks/#section=test&runid=592cab59-a9db-463b-a9c9-33d2f9484e92&hw=ph&test=db) for example.

Looking at the "single query" results, I first suspected that it was caused by there being not enough memory for the JVM runtime, so I added some logging code that prints the memory usage using `Runtime.totalMemory` and `Runtime.maxMemory`. It showed that there was about 7 GB max memory available during the benchmark runs, and the program only used 400 MB to 1 GB. I then tried allocating a 4 GB array during the run to ensure that the memory was usable and it worked with no problem.

Then looking at the "JSON serialization" results again, I saw that "vertx-web-kotlinx" performs a lot worse in this test too, and decided that this is more likely to be the bottleneck. Therefore, the static serializers are provided explicitly and the performance is improved slightly as tested on my machine. (Also, see commit 315b4e3 for an attempt before.) I then copied the "JSON serialization" test code from "vertx-web-kotlin-coroutines" and ran the benchmark to see if there were other factors, such as project configuration differences, affecting the performance, and the answer was no. On my machine, the "JSON serialization" performance of "vertx-web-kotlinx" is about 80% - 85% of that of "vertx-web-kotlin-coroutines". And I think the bottleneck possibly lies in kotlinx.serialization serializing an object to a byte array first and then copying it to a Vert.x buffer.

Remove the broken tag in "vertx-web-kotlin-coroutines" BTW, which was added in commit e53e026, for the benchmark runs without problems now as I tested.

frameworks/Kotlin/vertx-web-kotlin-coroutines/benchmark_config.json

@@ -18,8 +18,7 @@
       "database_os": "Linux",
       "display_name": "vertx-web-kotlin-coroutines",
       "notes": "",
-      "versus": "vertx-web",
-      "tags": ["broken"]
+      "versus": "vertx-web"
     },
     "postgres": {
       "db_url": "/db",

frameworks/Kotlin/vertx-web-kotlinx/src/main/kotlin/MainVerticle.kt

@@ -20,7 +20,7 @@ import kotlinx.coroutines.launch
 import kotlinx.html.*
 import kotlinx.html.stream.appendHTML
 import kotlinx.serialization.Serializable
-import kotlinx.serialization.encodeToString
+import kotlinx.serialization.SerializationStrategy
 import kotlinx.serialization.json.Json
 import java.net.SocketException
 import java.time.ZonedDateTime
@@ -111,11 +111,13 @@ class MainVerticle(val hasDb: Boolean) : CoroutineVerticle() {
         putHeader(HttpHeaders.CONTENT_TYPE, "application/json")
     }
 
-    inline fun <reified T : Any> Route.jsonResponseHandler(crossinline requestHandler: suspend (RoutingContext) -> @Serializable T) =
+    inline fun <reified T : Any> Route.jsonResponseHandler(
+        serializer: SerializationStrategy<T>, crossinline requestHandler: suspend (RoutingContext) -> @Serializable T
+    ) =
         checkedCoroutineHandlerUnconfined {
             it.response().run {
                 putJsonResponseHeader()
-                end(Json.encodeToString(requestHandler(it)))/*.coAwait()*/
+                end(Json.encodeToString(serializer, requestHandler(it)))/*.coAwait()*/
             }
         }
 
@@ -127,16 +129,16 @@ class MainVerticle(val hasDb: Boolean) : CoroutineVerticle() {
     }
 
     fun Router.routes() {
-        get("/json").jsonResponseHandler {
+        get("/json").jsonResponseHandler(Serializers.message) {
             jsonSerializationMessage
         }
 
-        get("/db").jsonResponseHandler {
+        get("/db").jsonResponseHandler(Serializers.world) {
             val rowSet = selectWorldQuery.execute(Tuple.of(randomIntBetween1And10000())).coAwait()
             rowSet.single().toWorld()
         }
 
-        get("/queries").jsonResponseHandler {
+        get("/queries").jsonResponseHandler(Serializers.worlds) {
             val queries = it.request().getQueries()
             selectRandomWorlds(queries)
         }
@@ -178,7 +180,7 @@ class MainVerticle(val hasDb: Boolean) : CoroutineVerticle() {
             }
         }
 
-        get("/updates").jsonResponseHandler {
+        get("/updates").jsonResponseHandler(Serializers.worlds) {
             val queries = it.request().getQueries()
             val worlds = selectRandomWorlds(queries)
             val updatedWorlds = worlds.map { it.copy(randomNumber = randomIntBetween1And10000()) }

frameworks/Kotlin/vertx-web-kotlinx/src/main/kotlin/Serializers.kt

@@ -0,0 +1,7 @@
+import kotlinx.serialization.serializer
+
+object Serializers {
+    val message = serializer<Message>()
+    val world = serializer<World>()
+    val worlds = serializer<List<World>>()
+}