Add optional auto-subscription parameters to APIs

[abitmore]

User Story
These APIs may auto-subscribe to querying objects:

• get_accounts(vector)
• get_assets(vector)
• get_objects(vector)
• lookup_accounts(string, uint32_t)

The BitShares UI team wants to be able to opt-out auto-subscription for all these APIs, just like the behavior of get_full_accounts API (bitshares/bitshares-ui#2612).

Since bitshares-core now supports optional parameters for API calls (bitshares/bitshares-fc#126), the feature can be implemented by adding an optional parameter to the APIs without break existing client applications.

Originally discussed in #777. Part of #1766.

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)
• UX (the User Experience)

CORE TEAM TASK LIST

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

[oxarbitrage]

Closed by #1868

Documentation may be needed somewhere but not mandatory as commands involved will keep working as before.