Draft: Prevent concurrent Ruby to use DB and Gitaly

What does this MR do and why?

This MR implements preventing concurrent Ruby from using DB and Gitaly.

Running external methods can allocate I/O bound resources (like PostgreSQL connection or Gitaly). This is forbidden when running within a concurrent Ruby thread, for example async HTTP requests provided by the gitlab-http gem.

Related to #427701 (closed)

Alternative: !136294 (merged).

MR acceptance checklist

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