WebServer: handle initial read/close timeouts asynchronously

If there is a delay before data becomes available on a new connection
then the handler function blocks for up to 1 second, and there's another
wait on connection close for up to 2 seconds.

This doesn't make the whole server process asynchronous (there is another
delay between headers/data) but it helps when there are other events
that need attention while handling web requests.

libraries/ESP8266WebServer/src/ESP8266WebServer.h

@@ -29,6 +29,7 @@
 enum HTTPMethod { HTTP_ANY, HTTP_GET, HTTP_POST, HTTP_PUT, HTTP_PATCH, HTTP_DELETE, HTTP_OPTIONS };
 enum HTTPUploadStatus { UPLOAD_FILE_START, UPLOAD_FILE_WRITE, UPLOAD_FILE_END,
                         UPLOAD_FILE_ABORTED };
+enum HTTPClientStatus { HC_NONE, HC_WAIT_READ, HC_WAIT_CLOSE };
 
 #define HTTP_DOWNLOAD_UNIT_SIZE 1460
 #define HTTP_UPLOAD_BUFLEN 2048
@@ -151,6 +152,8 @@ protected:
   WiFiClient  _currentClient;
   HTTPMethod  _currentMethod;
   String      _currentUri;
+  HTTPClientStatus _currentStatus;
+  unsigned long _statusChange;
 
   RequestHandler*  _currentHandler;
   RequestHandler*  _firstHandler;