Fixed caching

[Thorium]

There are 2 issues with the current caching. (Yes, I know it's taken from FSharp.Data.)

1. The cache invalidation is not very good, but as we know it's endless rabbit hole, I'm not fixing it right now.
2. The ConcurrentDictionary is used as normal dictionary, losing the benefit of it being ConcurrentDictionary. That is fixed with this PR. For more details, see: http://www.fssnip.net/7Qr/title/Correct-and-Incorrect-Way-to-Use-ConcurrentDictionary

[Thorium]

This might or might not fix the issue of #150

[Thorium]

What's wrong with macOS CI ? It says CI failed, but don't give any details.

[sergey-tihon]

Thank you @Thorium !
This fix released in v0.10.3

[Thorium]

I think this is not enough. This is interesting article:
https://andrewlock.net/making-getoradd-on-concurrentdictionary-thread-safe-using-lazy/

Now, the question is: is the adding types so resource consuming operation, that it would be better to lock other threads meanwhile?

[Thorium]

Added new PR #155 for that.