Use IndexedMerkleMap for OffchainState

[mitschabaude]

This follows #1666 and #1671 and uses the new IndexedMerkleMap for OffchainState.

Also:

• Fixes IndexedMerkleMap.clone() (cherry-picked in previous PR)
• Adds IndexedMerkleMap.{overwrite, overwriteIf}

TODO:

• [x] Make tree height, actions per update and actions per batch parameters that can be overriden
• [x] Fix proving when there are multiple settlement proofs in a row