[Audit Fix] Make sybil attacks a bit harder

[dmvt]

Type

• Enhancement
• Maintenance
• Refactor

Description

To make slow sybil attacks of the DAO more expensive, do not allow purchasing less than maxLambdaPurchase

Definition of Done

• ElasticDAO.join no longer accepts a _deltaLambda value
• ElasticDAO.join is updated to use maxLambdaPurchase as the value of _deltaLambda
• Tests are updated accordingly