Add threading model images and some content for user-book

[caojiele]

What is the purpose of the change

#30 apache/incubator-dubbo-website#49 According to the description of this a few problems, and his Pre, summarize and submit.

Brief changelog

modified: dubbo-user-book-en/demos/thread-model.md modified: dubbo-user-book/demos/thread-model.md modified: dubbo-user-book-en/sources/images/thread-model.png modified: dubbo-user-book/sources/images/thread-model.png

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