Don't re-use ReflectData in AvroCoder

[clairemcginty]

Addresses concurrency issue with encoding Avro records at scale -- we suspect the root cause is concurrent access of the shared ReflectData singleton.

[codecov]

Codecov Report

Merging #4951 (86db7bd) into main (0429f43) will decrease coverage by 0.02%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 86db7bd differs from pull request most recent head c26d0fb. Consider uploading reports for the commit c26d0fb to get more accurate results

@@            Coverage Diff             @@
##             main    #4951      +/-   ##
==========================================
- Coverage   63.10%   63.09%   -0.02%     
==========================================
  Files         282      282              
  Lines       10649    10646       -3     
  Branches      765      769       +4     
==========================================
- Hits         6720     6717       -3     
  Misses       3929     3929              

Files Changed	Coverage Δ
...com/spotify/scio/coders/instances/AvroCoders.scala	85.71% <100.00%> (-0.96%)	⬇️