[WIP/RFC] Follow chains of fixits

[bstaletic]

Sometimes resolving a fixit (eventually) results in more than one fixit. In those cases, ycmd will send the first one and set keep_going to True.
We can't reuse request data, because we need to inform ycmd about the updated state of the buffers.

PR Prelude

Thank you for working on YCM! :)

Please complete these steps and check these boxes (by putting an x inside
the brackets) before filing your PR:

• I have read and understood YCM's CONTRIBUTING document.
• I have read and understood YCM's CODE_OF_CONDUCT document.
• I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.
• I understand my PR may be closed if it becomes obvious I didn't
  actually perform all of these steps.

Why this change is necessary and useful

This is the client counterpart to ycm-core/ycmd#1760

---

This change is