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

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a GITHUB_issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a GITHUB issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like Fix UnknownException when host config not exist #XXX. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Test your code locally by running docsite start, and make sure it works as expected.
• Make sure no files under build directory is added.
• If this contribution is large, please follow the Software Donation Guide.