Added the ClientThreadsRespecter sync primitive.

It does. I added another, higher-order, Client for the whole object that boosts the ref count from 0 to 1 for the lifetime of the whole ClientThreadsRespecter.

https://github.com/dkorolev/tailproduce/commit/f8a8ba276cdaf3a2eca2ac3a2a22f82e789ad5b1