Add comment_rewards object in the `social_network`-plugin

There are 4 non-consesus fields about rewards in the comment_object which are used only for API objects:

To minimize memory usage for small nodes the following steps should be done:

[x] Create a new structure comment_reward_object in the social_network-plugin with fields:
comment (comment_object_id_type)
total_payout_value (total in GBG)
author_rewards (total in GOLOS)
author_gbg_payout_value (part in GBG)
author_golos_payout_value (part in GOLOS)
author_gests_payout_value (part in GESTS)
beneficiary_payout_value (GBG)
beneficiary_gests_payout_value (GESTS)
curator_payout_value (GBG)
curator_gests_payout_value (GESTS)
[x] Extend the author_reward_operation with the field author_rewards
[x] Add initialization of comment_reward_object from the author_reward_operation
[x] Add initialization of rewards information in API objects from the comment_reward_object
[x] Remove initialization of rewards fields from the database::cashout_comment_helper
[x] Remove 4 fields with rewards from the comment_object
[x] Add information about GETS, GOLOS, GBG pending parts in the discussion_helper::set_pending_payout
[x] Add configuration flag store-comment-rewards in the social_network-plugin. Default value is true.
[x] Fix unit-tests