We found that when a lot of sink writes happen concurrently through `Execution`s using `JobTest` you can observe data race for `Buffer`s of tuples created.

I've created a test to show this - if you do writes to 100 sinks in parallel it breaks internals of mutable map where we store tuple buffers for each mocked source and make test infinite.

I've added a lock to fix this race condition.

This also includes fix for Ruby's version on CI.