[TEST] Stress Update/Delete

Create the test index with merging disabled so deletes don't get merged away
this was breaking the version calculation.
More lenient type parsing in histo/cardinality aggs
closes #6948
closes #6893
[TEST] Stress update/delete

Reduce top end of scaling parameters to avoid OOMs on nightly runs.
[Discovery] remove unneeded cluster state serialization during cluster join process

At the moment we serialize the cluster state in JoinResponse and ValidateJoinRequest. However this state is not used anywhere and can be removed to save on network overhead

Closes #6949
Test: Set number_of_replicas to zero so that wait_for_green succeeds in search.aggregations
[Discovery] fix MembershipAction.sendJoinRequestBlocking should now expect an empty response

The change introduced in #6949 (do not serialize the cluster state) also means master now responds with an empty response rather then a JoinResponse. However, sendJoinRequestBlocking still expected a JoinRequest.