User callbacks for send failures

[Brian-1402]

Notes to resolve:

• Had to add E::Error: Send + 'static in multiple places in actor/src/send.rs to resolve the error of:

  <E as tokio_util::codec::Encoder<M>>::Error cannot be sent between threads safely
  

  But I'm still not sure what caused this error in the first place.
• For Retry, msg gets cloned even for successful sends. Bad for efficiency.
  • But confused how to resolve, coz how to predict whether the msg send will succeed or not? If send fails, then the initial attempt shouldn't have consumed msg coz now msg cannot be recovered to use in retry.
  • Can use Arc, but unsure of the decision for Arc overhead for every single msg sent.
  • Solved: recover the sent failed msg from Result::Error response
• Retry can be extended to also include timeout delay
  • Done.

Other decisions:

• Default choice is Retry 5 times with 100 ms delay.
• Callback choice affects both msg send failure and actor resolve failure.

Closes #56

[satyamjay-iitd]

1. Error can be Send + 'static, its fine.
2. You dont need to clone; The error that is returned returns the message that you tried to send. https://docs.rs/tokio/latest/tokio/sync/mpsc/struct.Sender.html#method.send
3. Add it in the enum and leave todo!() in the handler

[Brian-1402]

2. You dont need to clone; The error that is returned returns the message that you tried to send. https://docs.rs/tokio/latest/tokio/sync/mpsc/struct.Sender.html#method.send

Cool this seems to work for local sender, but I'm confused for remote sender (FramedWrite). Apparently it depends on the encoder to make the msg recoverable? Is adding a trait impl for the encoder to recover msg the right direction? @satyamjay-iitd