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.)
- [x] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [x] 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

Start by locating BitShares Core's current secp256k1-zkp dependency and integration, then compare its version and changes with the upstream ElementsProject/secp256k1-zkp repository. Identify affected security and maintainability checks before updating the dependency; done means the newer version is integrated and the existing project validation passes.

Written by the indexing model from the issue text.