2.2.4 release preparations

[akarnokd]

Version 2.2.4 - November 23, 2018 (Maven)

API changes

• Pull 6278: Add Maybe/Single/Completable materialize operator,
• Pull 6278: Add Single.dematerialize(selector) operator.
• Pull 6281: Add Flowable/Observable dematerialize(selector) operator.

Bugfixes

• Pull 6258: Fix cancel/dispose upon upstream switch for some operators.
• Pull 6269: Call the doOn{Dispose|Cancel} handler at most once.
• Pull 6283: Fix Observable.flatMap to sustain concurrency level.
• Pull 6297: Fix refCount eager disconnect not resetting the connection.

Documentation changes

• Pull 6280: Improve the package docs of io.reactivex.schedulers.
• Pull 6301: Add missing onSubscribe null-checks to NPE docs on Flowable/Observable subscribe.
• Pull 6303: Fix incorrect image placement in Flowable.zip docs.
• Pull 6305: Explain the non-concurrency requirement of the Emitter interface methods.
• Pull 6308: Explain the need to consume both the group sequence and each group specifically with Flowable.groupBy.
• Pull 6311: Explain that distinctUntilChanged requires non-mutating data to work as expected.

Wiki changes

• Pull 6260: Add generate examples to Creating-Observables.md.
• Pull 6267: Fix Creating-Observables.md docs stlye mistake.
• Pull 6273: Fix broken markdown of How-to-Contribute.md.
• Pull 6266: Update Error Handling Operators docs.
• Pull 6291: Update Transforming Observables docs.

Other changes

• Pull 6262: Use JUnit's assert format for assert messages for better IDE interoperation.
• Pull 6263: Inline SubscriptionHelper.isCancelled().
• Pull 6275: Improve the Observable/Flowable cache() operators.
• Pull 6287: Expose the Keep-Alive value of the IO Scheduler as System property.
• Pull 6321: Fix Flowable.toObservable backpressure annotation.