Add more database APIs to query for credit deals

**User Story**

As an API user / client app developer I would like to have more APIs to query for credit deals, so that it would be easier to develop related features in client app:
* query by both offer ID and borrower
* query by both offer owner and borrower

It is good to have APIs to query for summaries of data above when the amount of data in the results is large, although it is unlikely at this moment.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.
- [x] 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 (please add below)

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

The issue names no files, tests, or entry points. Start by locating the existing credit-deal query APIs and their underlying data structures, then review how API results are filtered and paginated. Done means exposing queries for offer ID plus borrower and offer owner plus borrower, with summary responses evaluated if large results require them.

Written by the indexing model from the issue text.