Add optional subscribe param to some database APIs

[abitmore]

PR for #1819.

These APIs are updated:

• added a subscribe parameter with type optional<bool>
  • get_objects ( vector ids, optional<bool> subscribe )
  • get_assets ( vector symbols_or_ids, optional<bool> subscribe )
  • get_accounts ( vector names_or_ids, optional<bool> subscribe )
  • lookup_accounts ( string lower_bound, int limit, optional<bool> subscribe )
  • get_htlc ( htlc_id id, optional<bool> subscribe )
• changed type of subscribe parameter to optional<bool>
  • get_full_accounts ( vector names_or_ids, optional<bool> subscribe )

* @param subscribe @a true to subscribe to the queried objects;
*                  @a false to not subscribe;
*                  @a omit or null to subscribe or not subscribe according to current
*                     auto-subscription setting (see @ref set_auto_subscription)

[abitmore]

Added test cases. This PR is now ready for review.

[abitmore]

@sschiessl-bcp and @startailcoon: you may have interest.