Running tests and exec-ing executables should be possible while build server is running

[gridbugs]

Desired Behavior

I often run dune test, dune build @test/<test> or dune exec <exe> while the build server is running, get the error A running dune instance has locked the build directory., kill the build server, and then run my original command again. I would rather not have to do this. I understand that these commands all build some files before running them so perhaps they could just take the latest versions of the files they need if they notice that the build server is running, possibly after waiting for the current build to complete.

Example

Inside a dune project, in one terminal I want to be able to run:

dune build --watch

And in a second terminal in the same project I want to be able to run:

dune exec path/to/my/exe

...or...

dune test

...or...

dune build @test/name_of_test

...without the fact that the build directory is locked preventing the command from running.

[Alizter]

The solution we ought to do is just connect to the existing RPC server when we do a build. @rgrinberg what was needed for this to happen again?

[rgrinberg]

We need to rally the errors and progress from the server to the client