Allow Lockable to be passed custom column, key, and Postgres advisory lock/unlock function

[bensheldon]

• Eventual intention is working towards locking on the ActiveJob ID rather than the GoodJob Job ID.
• Also supports work of Custom advisory locks to prevent certain jobs from being worked on concurrently? #206.

[reczy]

Hey @bensheldon - thoughts on using a single multi-line string in the :advisory_lock scope rather than a series of arel commands? My opinion is that reading the sql in its entirety makes it a bit easier to grasp what's going on here.

[bensheldon]

@reczy I agree that using Arel has the regex problem of "now you have two problems". I have a few reasons to use Arel:

• I'm personally leaning into Rails and ActiveRecord, which heavily involves Arel.
• I'd like to extract the Lockable module into its own gem, and I'd like to retain some flexibility in what it can do and expect and decompose.

...that being written, I also don't want Arel to be a blocker for people contributing.

The sql query is documented in the tests:

good_job/spec/lib/good_job/lockable_spec.rb

Lines 21 to 41 in e83d274

	it 'generates appropriate SQL' do
	query = model_class.where(priority: 99).order(priority: :desc).limit(2).advisory_lock
	expect(normalize_sql(query.to_sql)).to eq normalize_sql(<<~SQL.squish)
	SELECT "good_jobs".*
	FROM "good_jobs"
	WHERE "good_jobs"."id" IN (
	WITH "rows" AS #{'MATERIALIZED' if model_class.supports_cte_materialization_specifiers?} (
	SELECT "good_jobs"."id"
	FROM "good_jobs"
	WHERE "good_jobs"."priority" = 99
	ORDER BY "good_jobs"."priority" DESC
	)
	SELECT "rows"."id"
	FROM "rows"
	WHERE pg_try_advisory_lock(('x' || substr(md5('good_jobs' || "rows"."id"::text), 1, 16))::bit(64)::bigint)
	LIMIT 2
	)
	ORDER BY "good_jobs"."priority" DESC
	SQL
	end

...but I can better document what's happening in the code itself too.

Also, thank you for commenting on this PR. I was a little premature in opening it (I gotta get better at "Draft" PRs)

[bensheldon]

I extracted the FOR UPDATE SKIP LOCKED change to #274.