See #5868

There is a problem with the existing 1.x Observable.groupBy overload with an evicting map factory where depending on the eviction behaviour of the supplied map factory, groups may not be completed and may leak memory. The problem extends to the method signature in that the eviction action should report the evicted GroupedObservable not the corresponding key.

This PR

• fixes the existing method at the expense of doubling up the internal map groups ( into groupsCopy) so that we can always successfully lookup the evicted group from the groupsCopy map. Evictions from groups are mirrored in groupsCopy after the lookup.
• deprecates the problematic overload indicating that it uses more memory than the preferred new overload
• adds a new method groupBy that has the corrected signature and offers bufferSize and delayError parameters for a bit more flexibility and to differentiate the erased signature from the deprecated method
• adds tests that were ported from the 2.x tests