Various metadata-related features and improvements:

- Added support for new "NowMs" field that holds up to where no new packets are guaranteed to show up, in order to lower latency.
- Added support for JSON tracks over all TS-based protocols (input and output)
- Added support for AMF metadata conversion to JSON (RTMP/FLV input)
- Fixed MP4 input subtitle tracks
- Generalized websocket-based outputs to all support the same commands and run the same core logic
- Added new "JSONLine" protocol that allows for generic direct line-by-line ingest of subtitles and/or JSON metadata tracks over a TCP socket or console standard input.

lib/util.cpp

@@ -840,6 +840,11 @@ namespace Util{
     return true;
   }
 
+  /// Returns true if the given field exists.
+  bool RelAccX::hasField(const std::string & name) const{
+    return (fields.find(name) != fields.end());
+  }
+
   /// Returns the (max) size of the given field.
   /// For string types, returns the exact size excluding terminating null byte.
   /// For other types, returns the maximum size possible.