Clean up x-proton headers in Rest API spec and Proton SDKs

**Describe what enhancement you'd like to have**
We have some leftovers in the REST API spec regarding `x-proton-` header and we probably used `x-proton-` header in (Java) SDKs. Fix them to use `x-timeplu-` header. 

https://github.com/timeplus-io/proton/tree/develop/spec/rest-api
https://github.com/timeplus-io/proton-java-driver