firebase-ios-sdk

C++ migration: use C++ serializer for communicating with the local store (#4116)

After this PR is merged, the Objective-C serializers can be completely removed from the project. Also, this PR changes the `Message` class quite a bit; it's probably in its final form now.

I did some performance testing locally and came to the conclusion that replacing Objective-C serializers with their C++ versions brings no performance downgrade; actually, there seems to be a very slight improvement. I tested writing large batches and reading large collections:
* both are network-dominated; wall time is about twice longer than CPU time;
* serialization represents a pretty small percentage of the total CPU time;
* for writing, C++ serialization is a little more efficient, taking ~6% of total CPU time, compared to ~8% in the Objective-C version. I've seen similar results for reading.

All integration tests pass, under Address Sanitizer as well.