increase min memory to 15MB for indexing

[PSeitz]

With tantivy 0.20 the minimum memory consumption per SegmentWriter increased to
12MB. 7MB are for the different fast field collectors types (they could be
lazily created). Increase the minimum memory budget from 3MB to 15MB to avoid single doc segments.

Change memory variable naming from arena to budget.

closes #2156

[codecov-commenter]

Codecov Report

Patch coverage: 95.38% and project coverage change: -0.03% ⚠️

Comparison is base (1932513) 94.42% compared to head (9b5e9b1) 94.39%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2176      +/-   ##
==========================================
- Coverage   94.42%   94.39%   -0.03%     
==========================================
  Files         322      322              
  Lines       63104    63122      +18     
==========================================
  Hits        59583    59583              
- Misses       3521     3539      +18     

Files Changed	Coverage Δ
src/functional_test.rs	0.00% <0.00%> (ø)
src/query/boolean_query/boolean_query.rs	95.72% <ø> (ø)
src/query/disjunction_max_query.rs	82.14% <ø> (ø)
src/indexer/index_writer.rs	96.84% <94.73%> (+0.02%)	⬆️
src/collector/count_collector.rs	100.00% <100.00%> (ø)
src/collector/facet_collector.rs	98.90% <100.00%> (ø)
src/collector/histogram_collector.rs	98.53% <100.00%> (ø)
src/collector/mod.rs	54.63% <100.00%> (ø)
src/collector/multi_collector.rs	99.41% <100.00%> (ø)
src/core/index.rs	87.43% <100.00%> (ø)
... and 7 more

... and 3 files with indirect coverage changes

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