Fix cancel_all_subscriptions API

[ryanRfox]

USER STORY

As an API Client the the set_subscribe_callback function will always clear filters.

Current implementation calls set_subscribe_callback( std::function<void(const fc::variant&)>(), true)

Using true as the second parameter here meant to clear filters according to the API documentation (reference needed), but the documentation is wrong. The set_subscribe_callback function will always clear filters, and its second parameter is for something else. Setting it to true may trigger an exception.

TASK LIST

• [ ] Test case
• [ ] Fix code
  • [ ] Rewrite cancel_all_subscriptions to do real cleanups
  • [ ] Refactor it and set_subscribe_callback to call same cleanup function rather than having same code in both functions.
• [ ] QA/Testing
• [ ] Update documentation
  • [x] Documentation updated in Develop branch (reference needed)

[ryanRfox]

Original Issue text: "For clients, it's not broken. However, the implementation need to be improved. It now calls set_subscribe_callback( std::function<void(const fc::variant&)>(), true). I think using true as the second parameter here meant to clear filters according to the API document, but the document was wrong. The set_subscribe_callback function will always clear filters, and its second parameter is for something else, setting it to true may trigger an exception."