Move non-consensus indices (by_last_update, by_author_last_update) from golos::chain::comment_index to social_network-plugin

There are two non-consensus indices (by_last_update and by_author_last_update) in the golos::chain::comment_index, which are protected by the precompile condition IS_LOW_MEM.

Steps to remove IS_LOW_MEM mode:

[x] Create a new structure comment_last_update_object in the social_network-plugin with fields: comment (comment_object_id), parent_author, author and last_update
[x] Remove fields last_update and active from the comment_object
[x] Move initialization of last_update and active from evaluators to the social_network-plugin
[x] Create a new index for the comment_last_update_object with two indicies: by_last_update and by_author_last_update
[x] Change type of last_update in API objects (comment_api_object and discussion) to optional
[x] Add initialization of moved fields in API objects (comment_api_object and discussion) to the discussion_helper
[x] Add a configuration flag store-comment-last-update. The default value is true
[x] Fix unit tests

Ссылка на github