Add sensible SQLite default Pragmas (with ability to override)

[cllns]

Summary

Adds Hanami::DB::SQLite::Pragmas, a small value class that holds a set of SQLite pragmas with sensible defaults, accepts user overrides, validates pragma names against the linked SQLite, and exposes #connect_sqls, which is an array of PRAGMA statements suitable for Sequel's per-connection :connect_sqls hook.

This matches @fractaledmind's recommendations, which landed in Rails:

pragma	value
foreign_keys	true
journal_mode	:wal
synchronous	:normal
mmap_size	128 MiB
journal_size_limit	64 MiB
cache_size	2000 pages

Usage

# Just the defaults
pragmas = Hanami::DB::SQLite::Pragmas.new

# Override individual values; defaults still apply for the rest
pragmas = Hanami::DB::SQLite::Pragmas.new(overrides: {synchronous: :full})

# Drop the defaults entirely
pragmas = Hanami::DB::SQLite::Pragmas.new(clear_defaults: true, overrides: {…})

# Wire into Sequel
Sequel.connect(url, connect_sqls: pragmas.connect_sqls)

To try these out in a full-stack hanami app, use:

config.gateway(:default) do |gateway|
  gateway.connection_options(connect_sqls: Hanami::DB::SQLite::Pragmas.new.connect_sqls)
end

And verify with Hanami.app["db"].fetch("PRAGMA journal_mode").first.

In my app, benchmarking the 23 endpoints I have with a 'realistic' read-heavy load (on a file-based SQLite db), reads improved ~10% on average, and 3-25% faster for p95 (mostly 10-15%). For writes, most writes were much faster (30-50%), while some degraded slightly (by a small amount).

Importantly, there were some really bad p95 values of 5s+ which are gone now, which matters even more than the modest/significant performance increase above.

I'll open a PR to make this the default experience in hanami/hanami once this lands, so all SQLite users will benefit from it.

Design notes

• Name validation runs against the linked SQLite's own pragma_list (via the pragma_pragma_list table-valued function), so newer pragmas are recognised automatically without a gem upgrade.
• busy_timeout was included in @fractaledmind's conference talk. Will add this a follow-on PR to update Sequel config for SQLite alongside other driver concerns (e.g. integer_booleans), not in a class named Pragmas.
• Per-connection by design. PRAGMA state lives on the connection, not the database. #connect_sqls returns an array of SQL strings that Sequel applies via its :connect_sqls option to every new physical connection the pool opens. An :after_connect callback would have been the wrong abstraction: Sequel passes the raw adapter handle (SQLite3::Database), not a Sequel::Database, and a one-shot call against a single Sequel::Database would only configure whichever pool member happened to handle that call.
• Always validates names. I originally had an option where users could pass in validate: false, based on when I was validating against the sqlite3 gem, but I switched the method to call a query, and I can't think of any reason why anyone would want to skip validation, except one: using a very old SQLite version. The PRAGMA LISt was added in SQLite 3.20.0 which was released August 2017. I highly doubt anyone will be connecting to a DB that old with Hanami, but I'm open to putting it back in as an escape valve. It does open a connection to :memory: on call but that's in every SQLite compilation and is extremely fast, plus it's only run once.