Optimize the statement check for a non-discarded database

[flavorjones]

Closes #564 which pointed out a performance regression from #558.

This also restores the ability (now tested!) to call Database#close successfully and defer its cleanup until after Statements are closed, which was the promise of #557 and sqlite3_close_v2.

Running the benchmark @tenderlove wrote for #564:

#!/usr/bin/env ruby

require "bundler/inline"

gemfile do
  source "https://rubygems.org"
  gem "sqlite3", path: "."
  gem "benchmark-ips"
end

require "sqlite3"
require "benchmark/ips"

file = "/tmp/test.sqlite3"

begin
  File.unlink file
rescue
  nil
end

db = SQLite3::Database.new file
db.execute "create table foo (id int)"

1000.times do |i|
  db.execute "insert into foo (id) values (#{i})"
end

Benchmark.ips do |x|
  x.report("selecting") {
    db.execute("select * from foo").to_a
  }
end

db.close

On 480f3e7 after sqlite3_open_v2 was introduced by #557:

ruby 3.3.5 (2024-09-03 revision ef084cc8f4) [x86_64-linux]
Warming up --------------------------------------
           selecting   533.000 i/100ms
Calculating -------------------------------------
           selecting      5.306k (± 1.3%) i/s  (188.47 μs/i) -     26.650k in   5.023715s

On 81ea485 after fork protection was introduced by #558, performance regressed:

ruby 3.3.5 (2024-09-03 revision ef084cc8f4) [x86_64-linux]
Warming up --------------------------------------
           selecting   329.000 i/100ms
Calculating -------------------------------------
           selecting      3.261k (± 3.4%) i/s  (306.66 μs/i) -     16.450k in   5.051880s

On 56d47a6 in this PR, performance is restored.

ruby 3.3.5 (2024-09-03 revision ef084cc8f4) [x86_64-linux]
Warming up --------------------------------------
           selecting   548.000 i/100ms
Calculating -------------------------------------
           selecting      5.476k (± 1.7%) i/s  (182.61 μs/i) -     27.400k in   5.004987s

[flavorjones]

@tenderlove I'd like to merge this and cut an RC2 today, but would love your eyes on this if you get a chance.

[tenderlove]

@flavorjones sorry was busy today. This is great though, thank you for fixing it!