Make y-rb thread-safe

[eliias]

Problem

y-rb is not thread-safe. Originally I thought there are no use cases where concurrency patterns need to be supported, but it is relatively easy to shoot yourself into the foot when using y-rb with ActionCable in Rails. In particular, the combination of update events (attach(…)) with worker pools cause some issues (I get segfaults here and then).

To do

• Set up a test case that somewhat deterministically fails within a multithreaded environment
• Evaluate blocking transaction usage with one of the primitives from https://github.com/ruby-concurrency/concurrent-ruby