Initial Ractor support

[sandlerr]

Allows users to create / open databases, send queries, and process ResultSets inside a Ractor. However, does not allow passing the gem's domain objects through the Ractor communication channels.

See #299 .

[sandlerr]

Is there any outstanding feedback here? Thanks

[tenderlove]

This seems fine to me. I haven't investigated everything we need to do to make C extensions Ractor safe but I think this is fine to start with!

[flavorjones]

Great, I'll rebase and merge it today or tomorrow.

[flavorjones]

So it seems like in Ruby 3.3, trying to pass a Database between ractors results in a Ractor::RemoteError being raised; but in earlier versions of Ruby it makes an implicit copy of the object. This means that the test_ractor_share_database test hangs.

I'm not sure if this is intentional behavior or not. @sandlerr or @tenderlove do you know?

Edit: to be explicit, the RemoteError is wrapping Ractor::Error: can not copy SQLite3::Database object.

[flavorjones]

OK, that exception started to get raised in ruby/ruby@ce47ee00 as a fix, so I'm just going to adapt the test to the different behaviors before and after 3.3.0.

[flavorjones]

The stress test hangs under Ruby 3.3 with as few as 3 ractor writers, and it's not clear to me why.