New Revision metadata and Snapshot

[osopardo1]

This PR solves #27 and moves the old concept of SpaceRevision into a new Revision case class. This Revision contains more configurable parameters like the desired cube size or the columns used for index:

case class Revision(
    id: RevisionID,
    timestamp: Long,
    desiredCubeSize: Int,
    indexedColumns: Seq[String],
    transformations: IndexedSeq[LinearTransformation]) 

Disclaimer: ID and timestamp are the same right now.

In order to provide Revision isolation on Analyze, Optimize and Write operations, RevisionData contains only the necessary information about a single revision, such as:

• Revision information: timestamp, desiredCubeSize, indexedColumns and space transformations
• Replicated Set: Set of cubes in a replicated state for the single Revision
• CubeWeights: Map of cubes and the maximum Weight values for the single Revision

For Read operation, we still have to query all available RevisionData provided by QbeastSnapshot.

Accordingly, some information is changed in Metadata:

 "configuration": {
      "qbeast.lastRevisionID": "1635320405236",
      "qbeast.revision.1635320405236": "{\"id\":1635320405236,\"timestamp\":1635320405236,\"desiredCubeSize\":3000000,\"indexedColumns\":[\"user_id\",\"product_id\"],\"transformations\":[{\"min\":1.89823355E8,\"max\":6.91766695E8,\"scale\":1.9922567355909134E-9},{\"min\":-2.8748538E7,\"max\":9.0249526E7,\"scale\":8.403498060270964E-9}],\"dimensionCount\":2}"
    }

[cugni]

I think we should have different revisionID from revisionTimestamp, and make it clear in the API when we refer to a specific revision and when we want all revision happened before of a specific point in time.