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.

README.md

@@ -936,7 +936,7 @@ svr.set_read_timeout(std::chrono::seconds(5));
 svr.set_keep_alive_timeout(std::chrono::seconds(10));
 ```
 
-The read timeout bounds the request line and headers as a whole, and each read of a request body.
+The read timeout bounds the request line and headers as a whole, and each read of a request body. An error response closes the connection. Any other response to a request whose body was left unread, such as a redirect from a pre-routing handler, keeps the connection only when the rest of the body is at most `CPPHTTPLIB_UNREAD_PAYLOAD_MAX_LENGTH` (256KB, `0` for no limit) and arrives within the read timeout, since the server has to read past it first.
 
 ### Set maximum payload length for reading a request body