(perf) pool queryStringLex to reuse bufio.Reader across query parses

[huynhanx03]

Summary

• Pool queryStringLex (including its bufio.Reader with 4096-byte internal buffer) via sync.Pool to avoid allocating a new instance on every parseQuerySyntax call
• Every query string parse, including sub-queries in boolean/conjunction/disjunction, goes through this path, making it a high-frequency allocation hotspot
• Eliminates repeated bufio.NewReader allocation by reusing pooled instances via bufio.Reader.Reset()

Changes

• search/query/query_string_lex.go: Add sync.Pool with getQueryStringLex() and putQueryStringLex() helpers. Original newQueryStringLex() unchanged for backward compatibility.
• search/query/query_string_parser.go: parseQuerySyntax() uses pooled lexer instead of allocating new one.
• search/query/query_string_parser_test.go: Updated BenchmarkLexer to exercise the pooled path.

Benchmark

goos: darwin
goarch: arm64
cpu: Apple M1

name            old ns/op   new ns/op   delta
BenchmarkLexer  1,682       1,233       -26.7%

name            old B/op    new B/op    delta
BenchmarkLexer  5,344       1,104       -79.3%

name            old allocs  new allocs  delta
BenchmarkLexer  54          52          -3.7%

Checklist

• Code compiles correctly and linting passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable

[huynhanx03]

Hi @capemox , could you review, thanks.

[capemox]

Hi @huynhanx03, thanks for your contribution! Could you also benchmark and compare performance on a multi-goroutine setup?

[huynhanx03]

Hi @capemox , could you review, thanks.

Changes

• search/query/query_string_parser_test.go: Added BenchmarkLexer_Parallel using b.RunParallel to exercise the pooled path under concurrent goroutine contention.

Benchmark

goos: darwin
goarch: arm64
cpu: Apple M1

name                     old ns/op   new ns/op   delta
BenchmarkLexer_Parallel  1,283       631.3       -50.8%

name                     old B/op    new B/op    delta
BenchmarkLexer_Parallel  5,344       1,104       -79.3%

name                     old allocs  new allocs  delta
BenchmarkLexer_Parallel  54          52          -3.7%

[coveralls]

coverage: 52.523% (+0.02%) from 52.502%
when pulling b50a318 on huynhanx03:dev
into 0d388d7 on blevesearch:master.

[huynhanx03]

Hi @capemox , could you review, thanks.

[huynhanx03]

Hi @abhinavdangeti , could you review and approve? Thanks!

[abhinavdangeti]

Thank you for this contribution @huynhanx03 .