Don't wait for the peer's close_notify on OpenSSL shutdown

tls::shutdown() on OpenSSL called SSL_shutdown() a second time to wait
for the peer's close_notify. An idle keep-alive client never sends one,
so closing its connection held the worker thread until the read timeout,
and Server::stop() waited for it. Send close_notify and return, as the
Mbed TLS and wolfSSL backends already do.

httplib.h

@@ -19341,11 +19341,11 @@ inline void shutdown(session_t session, bool graceful) {
 
   auto ssl = static_cast<SSL *>(session);
   if (graceful) {
-    // First call sends close_notify
-    if (SSL_shutdown(ssl) == 0) {
-      // Second call waits for peer's close_notify
-      SSL_shutdown(ssl);
-    }
+    // Send close_notify without waiting for the peer's. The connection is
+    // closed right after this, so a unidirectional shutdown is enough, and an
+    // idle peer that never answers would otherwise hold this thread until the
+    // read timeout. The other backends do not wait either.
+    SSL_shutdown(ssl);
   }
 }