disable linear codec for multivalue values

[PSeitz]

disable linear codec for multivalue values
don't materialize index column on merge
use simpler chain() variant

[codecov-commenter]

Codecov Report

Merging #1591 (d742275) into main (7905965) will increase coverage by 0.02%.
The diff coverage is 97.00%.

@@            Coverage Diff             @@
##             main    #1591      +/-   ##
==========================================
+ Coverage   93.75%   93.77%   +0.02%     
==========================================
  Files         252      252              
  Lines       46317    46380      +63     
==========================================
+ Hits        43423    43495      +72     
+ Misses       2894     2885       -9     

Impacted Files	Coverage Δ
src/fastfield/mod.rs	99.71% <ø> (ø)
src/indexer/sorted_doc_id_multivalue_column.rs	95.00% <94.44%> (-0.46%)	⬇️
src/fastfield/multivalued/mod.rs	98.45% <100.00%> (+1.22%)	⬆️
src/fastfield/multivalued/writer.rs	98.34% <100.00%> (-0.03%)	⬇️
src/fastfield/serializer/mod.rs	100.00% <100.00%> (ø)
src/indexer/merger.rs	98.82% <100.00%> (-0.01%)	⬇️
src/fieldnorm/writer.rs	96.96% <0.00%> (-1.52%)	⬇️
src/directory/mmap_directory.rs	89.86% <0.00%> (-0.54%)	⬇️
src/fastfield/multivalued/reader.rs	96.42% <0.00%> (+5.35%)	⬆️
... and 1 more

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