Add a class to pass SeqNr and snapshot payload in

[rtar]

The main question here is if we want to have payload as an Option or require it always to be filled.

This idea of Option was taken from a similar Event case class, but I am not sure it applies the same way to a snapshot.

[coveralls]

Pull Request Test Coverage Report for Build 7801971852

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 81.497%

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Files with Coverage Reduction	New Missed Lines	%
journal/src/main/scala/com/evolutiongaming/kafka/journal/HeadCache.scala	2	84.62%
eventual-cassandra/src/main/scala/com/evolutiongaming/kafka/journal/eventual/cassandra/ReplicatedCassandra.scala	7	90.37%

Totals
Change from base Build 7759385047:	-0.2%
Covered Lines:	3277
Relevant Lines:	4021

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💛 - Coveralls

[Z1kkurat]

I struggle to imagine a case where we'd need a snapshot without a state inside. It looks like a tombstone, but we don't need those for persistent storage such as Cassandra, do we? We either have a snapshot with a state, or we delete the row altogether

[rtar]

@Z1kkurat I copied from Event structure.

I now suspect it was added there as part of the c6d0543 commit to ensure backwards compatibility with the previously stored binary snapshot.

I will rework it to remove Option now.

CC: @dfakhritdinov

[rtar]

I have removed Option now (and also removed it from a large PR, which I am deconstructing into these smaller PRs, but you do not have to look there: #532).