SPV idea up for discussion

When I was in Amsterdam, I've had these *thoughts* that wouldn't let me rest until it made sense to me. So i stripped the thoughts apart and tried tackling them one at a time. The first thought that I finally got to put down in written is about SPV (Simple payment verification - the term doesn't fit well, but is known in the space). I wrote it all down in the bitshares-core wiki:

https://github.com/bitshares/bitshares-core/wiki/SPV

Given that discussion are best held via issues, I thought I should maybe put it here too.
Please feel free to edit/extend the wiki directly if you feel you can contribute there.

**User Story**
I want to be able to prove outside the BitShares Blockchain that a transaction is part of the blockchain with only little additional data provide (not the entire chain)

**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.)
- [x] Protocol (the blockchain logic, consensus, validation, etc.)
- [x] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [x] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start with the linked SPV wiki page and the user story. Define the protocol and security requirements, affected components, and test cases; done means the design is sufficiently specified for implementation and validation.

Written by the indexing model from the issue text.