[liblsl]: convert as much isolate passing to deeply-immutable / final / const as possible

In almost every instance of the isolate message passing, it is an unwanted side effect that copies of messages are used for passing data, while some of this is mitigated by reconstructing pointers from their addresses, an even better option is to change ALL instances (including pointer address messages) to use a common base message type that is deeply-immutable, sealed, etc..

this will not work with any list / map or enum instances (enums can be reconstructed from their value, but lists - e.g. channel data, is less likely to happen).