cursor-style pagination?

[roadscape]

currently with steemd all "posts" queries take a start (author,permlink) as a cursor to load successive pages from. hive just uses offset/limit. if we want to replicate existing APIs (nearly) 100% we would need to add the cursor option. this involves a bit more complexity because we'll need to perform a lookup on the column we're ordering by based on the author-permlink provided to know on which value to start. aside from the extra queries an upside is that seeking could be more efficient than simple offset; it's more infinite-scroll friendly; and, may be more consistent when result rows are 'moving' around.

for reference:

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;
   set<string>      select_authors; ///< list of authors to include, posts not by this author are filtered
   set<string>      select_tags; ///< list of tags to include, posts without these tags are filtered
   uint32_t         truncate_body = 0; ///< the number of bytes of the post body to return, 0 for all
   optional<string> start_author;
   optional<string> start_permlink;
   optional<string> parent_author;
   optional<string> parent_permlink;
};

[roadscape]

cursor-style pagination is being implemented as a requirement for legacy api (#19).