[RPC] Allow formatting while watch server is running

[ElectreAAS]

Desired Behavior

In one terminal dune build -w, in another dune fmt (or dune rpc fmt) and that works

Context

This would follow in the footsteps of #11808, #11712, #11840, and #11900

[rgrinberg]

As a start, you might want to just allow dune promote to work while RPC running. Doing fmt should be a natural extension of that.

[Alizter]

@rgrinberg That already seems to be the case today. dune promote doesn't care if the _build directory is locked when dune build -w is running.

[rgrinberg]

Unless things have changed recently, it does not work correctly. It's still possible for the two invocations of dune to step over each other while editing the promotion database. There needs to something to synchronize such access.

[ElectreAAS]

Solved by #12064, and to a lesser degree, #12010.