fix: reduce infix merge memory usage

[sanikolaev]

• store the infix arena and long UTF-8 keys in bounded segments
• compact short keys and one/two-checkpoint lists inline
• sort arena blocks and serialize them with a k-way merge
• honor min_infix_len throughout indexing and merge paths
• release builder storage as soon as infix entries are written
• add a local peak-RSS benchmark

Related issue: #4850

[sanikolaev]

Benchmarks show that this approach doesn't lead to significant improvements.

[sanikolaev]

Benchmarks show that this approach doesn't lead to significant improvements.

The command I tested with was:

sudo -u manticore manticore-load --cache-gen-workers=10   --wait   --cache-from-disk   --drop --port=9306   --threads=10   --batch-size=10000   --total=1600000   --init="CREATE TABLE t(title text) min_infix_len='3' index_exact_words='1' optimize_cutoff='1'; SET GLOBAL parallel_chunk_merges=1" --load="INSERT INTO t(id,title) VALUES(<increment>,'word<increment><string/24/24> word<increment><string/24/24> word<increment><string/24/24> word<increment><string/24/24> word<increment><string/24/24> word<increment><string/24/24> word<increment><string/24/24> word<increment><string/24/24>')"