Add an lsn header to the change messages

[samwillis]

The LSN is very useful for being able to "align" streams with each other. Currently the LSN is a part of the offset, but this is likely to change in the future. It would be useful if the LSN was added as a header to each change message.

[KyleAMathews]

And we're going to drop the offset from messages as well?

[samwillis]

@KyleAMathews

@icehaunter and @balegas have implied that the offset format will change and become storage dependent. I suspect a byte offset in our storage logs, if that happens we have a chicken and egg situation where the offset is a variable number of bytes in the JSON in the log, but is referencing its own offset in that log.... so I suspect we may want to drop it if thats the case.