fix(OverlayView): use ReactDOM.createPortal in React@^16

[urikphytech]

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[maciekbb]

Hello! Do you think it helps with performance issues described in #817?

I did a quick check and version from this PR seems to improve performance a lot. Time spent in OverlayView#getOffsetOverride went down from ~40% to ~3% when moving the map around in my specific use case.

[urikphytech]

Great news!
Since this repo is not currently maintained I am using a fork of it which includes this fix in it.
https://github.com/urikphytech/react-google-maps

[owap]

@maciekbb I don't think this solves the issues with getOffsetOverride; we tried cloning this fork, as well as making the changes in this PR by hand, and neither result in any reduction in the time we spend in getOffsetOverride (between 50 and 60% for us).

Did you find another solution that solves the issue? If so, we'd love to hear it!