Initial commit for thread-safe cb impl

[enbnt]

Start of examples where concurrency and
thread safety concerns are introduced. This
is meant to be an example for a blog post
where we can run jmh benchmarks concurrently
against each solution to determine locking/synchronization
overhead.

The blog should also discuss how https://www.baeldung.com/java-ring-buffer thread-safe
producer consumer is not guaranteed - the access and comparison to both read & write vars is not an atomic operation, which leaves us with some tricky synchronization to ensure accurate results.

TODO:

• test cases
• jmh perf benchmarks
• updated scala docs
• determine read/timeout semantics for sync version