MIMParser: simplify processing of single-xref records
MIMParser: simplify processing of single-xref records

Instead of a massive if-elsif cascade in which the only things different
were the source id passed to add_xref and the counter to increment,
create a generic object and choose the right source id / counter using
lookup hashes. The processing of two-record insertions takes advantage
of both counter selection and the generic object as well, albeit it is
not fully automated yet. Moved/removed entries only use generic counters
for now.

Ensembl/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl