Selectively use prefetch_related in RnaSequences
Selectively use prefetch_related in RnaSequences

* estimate the number of xrefs for each RNA
* prefetch_related only for RNAs with a small number of xrefs
* do not attempt to optimise entries with a large number of xrefs
letting Django hit the database one time for each xref

RNAcentral/rnacentral-webcode,RNAcentral/rnacentral-webcode,RNAcentral/rnacentral-webcode,RNAcentral/rnacentral-webcode