BibRank: size check on the input of 'get_refersto_hitset' and simular functions

* Limit DOS attacks with 'refersto' and similar keywords (e.g. 'refersto:year:0-->9999')
* Adds configuration variable CFG_BIBRANK_MAX_RECORDS_REFERSTO
* Adds configuration variable CFG_BIBRANK_MAX_RECORDS_CITEDBY
* Limits parameter size of 'get_refersto_by_list' and 'get_refersto_hitset' to CFG_BIBRANK_MAX_RECORDS_REFERSTO
* Limits parameter size of 'get_cited_by_list' and 'get_citedby_hitset' to CFG_BIBRANK_MAX_RECORDS_CITEDBY