[lldb-dap] Take two at refactoring the startup sequence. (#140331)

This is more straight forward refactor of the startup sequence that
reverts parts of ba29e60f9a2222bd5e883579bb78db13fc5a7588. Unlike my
previous attempt, I ended up removing the pending request queue and not
including an `AsyncReqeustHandler` because I don't think we actually
need that at the moment.

The key is that during the startup flow there are 2 parallel operations
happening in the DAP that have different triggers.

* The `initialize` request is sent and once the response is received the
`launch` or `attach` is sent.
* When the `initialized` event is recieved the `setBreakpionts` and
other config requests are made followed by the `configurationDone`
event.

I moved the `initialized` event back to happen in the `PostRun` of the
`launch` or `attach` request handlers. This ensures that we have a valid
target by the time the configuration calls are made. I added also added
a few extra validations that to the `configurationeDone` handler to
ensure we're in an expected state.

I've also fixed up the tests to match the new flow. With the other
additional test fixes in 087a5d2ec7897cd99d3787820711fec76a8e1792 I
think we've narrowed down the main source of test instability that
motivated the startup sequence change.

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