Posts in "@user/posts" are now stored as author/permlink.

[mvandeberg]

Fix and document get_replies_by_last_update
#445

[theoreticalbts]

• Needs FC_ASSERT( limit <= k ); for some fixed k (maybe 1000?)
• Should explicitly handle the case where both strings are empty (i.e. add else clause after else if)

[mvandeberg]

k should be 100 as used by discussion_query

struct discussion_query {
   void validate()const{
      FC_ASSERT( filter_tags.find(tag) == filter_tags.end() );
      FC_ASSERT( limit <= 100 );
   }

   string           tag;
   uint32_t         limit = 0;
   set<string>      filter_tags;
   optional<string> start_author;
   optional<string> start_permlink;
   optional<string> parent_author;
   optional<string> parent_permlink;
};

The else case is handled implicitly by

auto itr = last_update_idx.begin();

I can add an else case

else
   itr = last_update_idx.begin();

But that seems redundant...