Proposal: guided debugging

[dragonly]

When a developer is interested in TiDB, he/she may first clone the code and do some hack, then compile and debug for fun. We can make this a natural way of guide!

Here's the brief proposal, which could be detailed if anyone is interested in it:

On a stable version like v4.0, we add breakpoints to all the important code in a complete feature, like client connection handling/select query handing/tv storage RPC/etc. Then the user can choose a feature journey, and start debug using the breakpoints and some possible hints we pre-configured, and learn the code in a complete guided journey!