Make on_send support both v4 and v3 of stomp.py
Make on_send support both v4 and v3 of stomp.py

- under v4, on_send is past a Frame object only rather than a dictionary of headers
- on_send is now triggered on non APEL message messages (such as 'CONNECT' frames)
  and those messages do not have an empa-id.
- unused_body is required to be backward compatibaly with the stopm.py v3 method signiture of on_send,
  even though it's not used.

tofu-rocketry/ssm,stfc/ssm,tofu-rocketry/ssm,apel/ssm,apel/ssm,stfc/ssm