Redesign stack context structure

[jessesquires]

After talking with Daniel Eggert, I found that nesting parent/child contexts is non-optimal.

Move from the existing stack architecture, to the following:

• Remove mainContext as a child context of the backgroundContext
• Connect both backgroundContext and mainContext to the coordinator
• Merge via notification APIs

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Original design: 28b9e4e (what we want to go back to)
Refactor to parent-child: f0aa87c

Basically need to revert stack changes in f0aa87c to go back to 28b9e4e

[jessesquires]

Also:

http://stackoverflow.com/questions/29900193/core-data-parent-context-blocks-child/29918585#29918585

😄

[TimurBK]

I was actually quite surprised to find out that most of popular CoreData libraries use parent-child stack. On data set of hundreds it may add ~30% to fetch time(I got ~0.6s and by using main queue MOC directly connected to PSC I got 0.4s on iPhone 5).
Stack in Daniel's answer(probably can be called "classic") is performing much better. And if it's not enough, two separate persistent store coordinators connected to single store may help.

[jessesquires]

Definitely. After talking with Daniel, I learned that child contexts are really misunderstood and misused.

The best scenario is to use them as a scratch pad where you may want to completely throw out the objects.