add pagination and limit to get_key_references and get_account_references

UI team requesting this at https://github.com/bitshares/bitshares-core/pull/1749#discussion_r282520956

By the way, please fix the duplicate result issue (#1209), see the example below. Perhaps change the return data type from `vector` to `vector` or `vector`.

Review the get_key_references and get_account_references implementations and the UI discussion in pull request 1749. Then inspect issue 1209 and its duplicate-result example to understand the expected behavior. Done means both reference queries support pagination and limits, while duplicate results are eliminated and covered by the relevant tests.

Written by the indexing model from the issue text.