[fix-zip-observable] synchronize ZipObservable onComplete implementation

[lpetka]

ZipObservable implementations (all flavors Zip{2,3,4,5,6}Observable) uses shared state in multithreaded environment. There was a problem with underterministically non completing observables which was induced by the read of a non-volatile shared boolean without prior memory barrier.

I wasn't able to reproduce the problem on a current monix fork and the reason behind it is still unknown to me, although the major contributors might be:

• different java / scala version
• custom test executor with sophisticated ticking implementation

The problem is reproducing kinda well for a current DHCP implementation both in testing suites and production code (the executor provided for the observable is irrelevant there).

Playground: https://gitlab.avsystem.com/csp/dslam/dhcp/-/commit/ca08ccb46078c6f8e8cde3f1261d51527e2f39d1