Update secp256k1-zkp library to a more recent version

User Story

The secp256k1-zkp module used in the code base is too old. There have been many changes made in the upstream https://github.com/ElementsProject/secp256k1-zkp . As a developer I'd like that we update the module to a more recent version so that we are free from known security issues and etc.

Impacts Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.

• API (the application programming interface)
• Build (the build process or something prior to compiled code)
• CLI (the command line wallet)
• Deployment (the deployment process after building such as Docker, Travis, etc.)
• DEX (the Decentralized EXchange, market engine, etc.)
• P2P (the peer-to-peer network for transaction/block propagation)
• Performance (system or user efficiency, etc.)
• Protocol (the blockchain logic, consensus, validation, etc.)
• Security (the security of system or user data, etc.)
• UX (the User Experience)
• Other: code maintainability

CORE TEAM TASK LIST

• Evaluate / Prioritize Feature Request
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation