Tweak GENCODE loading performance

[federkasten]

This PR improves GENCODE's GTF/GFF3 loading performance with parallel processing using pmap, etc.

Test code:

(require '[varity.ref-gene :as rg])
(time (def rgidx (rg/index (rg/load-gff3 "/path/to/gencode.v38.annotation.gff3.gz"))))

Loading time:

• Before this PR: 205300.032914 msecs
• This PR: 49498.38981 msecs

[codecov]

Codecov Report

Merging #55 (aad6305) into master (25afd13) will increase coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   45.35%   45.65%   +0.30%     
==========================================
  Files          16       16              
  Lines        1958     1969      +11     
  Branches       60       60              
==========================================
+ Hits          888      899      +11     
  Misses       1010     1010              
  Partials       60       60              

Impacted Files	Coverage Δ
src/varity/ref_gene.clj	81.56% <100.00%> (+0.58%)	⬆️

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[k-kom]

It might be better to add this optional arg to both load-gtf and load-gff because currently these functions are easier to use. Or reimplement load-gencode to pick up an appropriate function based on f's extension (sorry my first implementation is not well designed).

[federkasten]

Thanks. I had missed it.

BTW, I think the GTF/GFF3 loader should use cljam's implementation (https://github.com/chrovis/cljam/blob/master/src/cljam/io/gff.clj). I am willing to do a complete rewrite on that.

[k-kom]

Thank you for quick revision. LGTM