Adding multithreaded example from issue #3054 (by airween)

- Rewritten to use C++ libModSecurity API and std::thread (instead of
  pthreads)

examples/multithread/multithread.cc

@@ -0,0 +1,68 @@
+#include <iostream>
+#include <thread>
+#include <array>
+
+#include <modsecurity/modsecurity.h>
+#include <modsecurity/transaction.h>
+#include <modsecurity/rules_set.h>
+
+static void process_request(modsecurity::ModSecurity *modsec, modsecurity::RulesSet *rules, int tid) {
+    std::cout << "Hello World! It's me, thread #" << tid << std::endl;
+
+    for(int i = 0; i != 1'000; i++) {
+        auto modsecTransaction = std::make_unique<modsecurity::Transaction>(modsec, rules, nullptr);
+
+        modsecTransaction->processConnection("127.0.0.1", 12345, "127.0.0.1", 80);
+        modsecTransaction->processURI(
+            "https://www.modsecurity.org/test?foo=herewego",
+            "GET", "1.1");
+
+        modsecTransaction->addRequestHeader("User-Agent",
+            "Basic ModSecurity example");
+        modsecTransaction->processRequestHeaders();
+        modsecTransaction->processRequestBody();
+
+        modsecTransaction->addResponseHeader("HTTP/1.1",
+            "200 OK");
+        modsecTransaction->processResponseHeaders(200, "HTTP 1.2");
+        modsecTransaction->processResponseBody();
+
+        modsecTransaction->processLogging();
+
+        std::this_thread::sleep_for(std::chrono::microseconds(100));
+    }
+
+    std::cout << "Thread #" << tid << " exits" << std::endl;
+}
+
+int main (int argc, char *argv[]) {
+    auto modsec = std::make_unique<modsecurity::ModSecurity>();
+    modsec->setConnectorInformation("ModSecurity-test v0.0.1-alpha (Simple " \
+        "example on how to use ModSecurity API");
+
+    char main_rule_uri[] = "basic_rules.conf";
+    auto rules = std::make_unique<modsecurity::RulesSet>();
+    if (rules->loadFromUri(main_rule_uri) < 0) {
+        std::cerr << "Problems loading the rules..." << std::endl;
+        std::cerr << rules->m_parserError.str() << std::endl;
+        return -1;
+    }
+
+    constexpr auto NUM_THREADS = 100;
+    std::array<std::thread, NUM_THREADS> threads;
+
+    for (auto i = 0; i != threads.size(); ++i) {
+        threads[i] = std::thread(
+            [&modsec, &rules, i]() {
+                process_request(modsec.get(), rules.get(), i);
+            });
+    }
+
+    std::this_thread::sleep_for(std::chrono::microseconds(10000));
+
+    for (auto i = 0; i != threads.size(); ++i) {
+        threads[i].join();
+    }
+
+    return 0;
+}