Add a class to pass SeqNr and snapshot payload in (#563)

* Add a class to pass SeqNr and snapshot payload in.

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.

* Remove option wrapper around a payload.

It was inherited from original `Event` code, but it seems that it
was only needed there to ensure backwards compatibility when introducing
JSON payloads and no longer necessary.

See c6d0543 for more details.

snapshot/src/main/scala/com/evolutiongaming/kafka/journal/Snapshot.scala

@@ -0,0 +1,13 @@
+package com.evolutiongaming.kafka.journal
+
+/** Represents a snapshot to be stored or loaded from a storage.
+  *
+  * @param seqNr
+  *   Snapshot sequence number.
+  * @param payload
+  *   Actual contents of a snapshot.
+  */
+final case class Snapshot[A](
+  seqNr: SeqNr,
+  payload: A
+)