Bound how much of an unread request body the server skips

After answering, with a status below 400, a request whose body the
handler left unread, the server read past the body to reuse the
connection up to the payload limit (100MB by default), with only each
read bounded by the read timeout, so a slow sender could hold a worker
for a long time. It now skips at most
CPPHTTPLIB_UNREAD_PAYLOAD_MAX_LENGTH (256KB, as Go's net/http does)
within the read timeout as a whole, and closes the connection
otherwise. An error response already closed the connection.

httplib.h

@@ -142,6 +142,12 @@
 #define CPPHTTPLIB_PAYLOAD_MAX_LENGTH (100 * 1024 * 1024) // 100MB
 #endif
 
+// How much of an unread request body the server skips to reuse the connection,
+// 0 for no limit
+#ifndef CPPHTTPLIB_UNREAD_PAYLOAD_MAX_LENGTH
+#define CPPHTTPLIB_UNREAD_PAYLOAD_MAX_LENGTH (256 * 1024) // 256KB
+#endif
+
 #ifndef CPPHTTPLIB_SSE_MAX_EVENT_LENGTH
 #define CPPHTTPLIB_SSE_MAX_EVENT_LENGTH (16 * 1024 * 1024) // 16MB
 #endif
@@ -15038,7 +15044,8 @@ Server::process_request(Stream &strm, const std::string &remote_addr,
 
   // The read timeout bounds the request line and headers as a whole, so a
   // client sending them a byte at a time cannot hold the worker any longer.
-  strm.set_max_timeout(read_timeout_sec_ * 1000 + read_timeout_usec_ / 1000);
+  auto read_timeout_msec = read_timeout_sec_ * 1000 + read_timeout_usec_ / 1000;
+  strm.set_max_timeout(read_timeout_msec);
 
   // Connection has been closed on client
   if (!line_reader.getline()) { return false; }
@@ -15422,14 +15429,18 @@ Server::process_request(Stream &strm, const std::string &remote_addr,
   // keep-alive. Without framing there is no body to drain — reading would
   // consume the next request (issue #2450). If the response has committed the
   // connection to close, there is no next request to protect. A body read in
-  // part cannot be drained from where it stopped, so the connection closes.
+  // part cannot be drained from where it stopped, so the connection closes,
+  // as it does when the body is longer than the drain limit or does not
+  // arrive within the read timeout.
   if (!req.body_consumed_ && detail::has_framed_body(req) &&
       !detail::has_header_token(res.headers, "Connection", "close")) {
+    strm.set_max_timeout(read_timeout_msec);
     int dummy_status;
     if (req.body_read_started_ ||
         !detail::read_content(
-            strm, req, payload_max_length_, dummy_status, nullptr,
-            [](const char *, size_t, size_t, size_t) { return true; }, false)) {
+            strm, req, CPPHTTPLIB_UNREAD_PAYLOAD_MAX_LENGTH, dummy_status,
+            nullptr, [](const char *, size_t, size_t, size_t) { return true; },
+            false)) {
       connection_closed = true;
     }
   }