Review all APIs, add new optional parameters and change some old parameters to optional

**User Story**
We'll soon support optional parameters for APIs (https://github.com/bitshares/bitshares-fc/pull/126), so we can update existing APIs to be more user-friendly.

For example, `get_call_orders` API now has two parameters, `asset` and `limit`, a user have to specify a limit, and can only query for the first page. If we change `limit` to optional and have a default value for it, users can simply call `get_call_orders(asset)` to fetch the top results; if we add an optional `start`, then users will be able to fetch next pages.

## 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 reviewing the optional-parameter support proposed in bitshares-fc#126 and the existing get_call_orders API. Inventory the other APIs and decide which parameters should become optional, their defaults, and how pagination such as start and limit should work. No source files or tests are named, so completion would require defined test cases and updated documentation for the chosen APIs.

Written by the indexing model from the issue text.