Delay constructing the config handle

[nirinchev]

This is a POC that delays constructing the native config until we actually get to open the Realm instance.

I think this is a cleaner approach for passing more complex data to native. We can use it for all config types (realm, sync, app) as well as for credentials.

[coveralls]

Pull Request Test Coverage Report for Build 2114345458

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.841%

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Totals
Change from base Build 2114305298:	0.0%
Covered Lines:	394
Relevant Lines:	429

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💛 - Coveralls

[nirinchev]

@blagoev updated API docs, but I'm not sure I have an idea for how a meaningful equality comparer would look like on the config, especially once we introduce callbacks for migration, compaction, and initial data.

@desistefanova I tend to agree with Lubo that the scheduler should be separate from the config, but it's possible I'm not seeing some elegant solution there. Feel free to ping me on slack if you feel strongly about it and we can look at it together.