dbutil: rewrite query placeholders without regexp

[jlucaso1]

With the SQLite dialect, mutateQuery runs positionalParamPattern.ReplaceAllString on every query that goes through LoggingExecable, so the regexp engine cost is paid on every single Exec/Query/QueryRow. This replaces it with a hand-rolled scan doing the same $N to ?N rewrite: queries without placeholders return the input string with zero allocations, and queries with placeholders cost one allocation for the rewritten string.

The new test checks the function against the old regexp on every combination of up to 4 characters from a relevant alphabet plus some realistic queries, so the behavior is provably identical. Benchmark on a typical INSERT:

BenchmarkReplacePositionalParams/scanner-12    166.4 ns/op    160 B/op    1 allocs/op
BenchmarkReplacePositionalParams/regexp-12     578.9 ns/op    514 B/op    7 allocs/op

I also considered caching rewritten queries, but dynamically built queries (IN clauses, mass inserts) would make the cache unbounded, and the scanner already gets it down to the single unavoidable allocation.

Before/after heap profiles from a whatsmeow ping/pong benchmark that uses this library with SQLite, viewable in speedscope: before, after. mutateQuery drops from 111MB to 22MB of allocations over the run, with the regexp machinery replaced by the single allocation of the rewritten string.