Improve createSettlement to fail attempts for continuous/gross models

[ggrg]

Goal:

As a Hub Operator
I want to make sure no settlement can be initiated for IMMEDIATE/GROSS models
so that only DEFERRED/NET models may result in a successful settlement create operation

Tasks:

• Set up various combination for settlement models (e.g. IMMEDIATE/GROSS, DEFERRED/GROSS) [@ggrg]
• Merge Node v2 upgrade from master into settlement-v2 for central-ledger and central-settlement. Also utilise upgraded ml-api-adapter and mojaloop/simulator for central-settlement integration tests run [@ggrg]
• Add unique constraint on settlementModels to prevent similar models (delay-granularity-interchange-ledgerAccountType-currency) [@ggrg]
• Improve createSettlement validation and make sure all of the above specified models fail with appropriate message [@ggrg]
• Fix unit tests

Acceptance Criteria:

• Inappropriate requests are rejected
• Designs are up-to date
• Unit Tests pass
• Integration Tests pass
• Code Style & Coverage meets standards
• Changes made to config (default.json) are broadcast to team and follow-up tasks added to update helm charts and other deployment config.
• A Settlement cannot be explicitly be initiated when he settlement model chosen is a IMMEDIATE/GROSS model

Pull Requests:

• feature/1209 Restrict Create Settlement central-ledger#606
• feature/1209 Restrict Create Settlement central-settlement#249

Follow-up:

• N/A

Dependencies:

• Update projects to use node 12.16.0 LTS #1175

Accountability:

• Owner: @ggrg
• QA/Review: @ggrg