Resources for Actions

This ticket originates from a discussion I've had with Andrey.

Problem

Some actions are quite expensive to execute and there's no way to express an estimate of the amount of resources they'll be using. For example, a test action can spin up a full database, a web server, and some integration tests as part of a single test. If a test action is this expensive, we need a way to tell dune not to run so many such expensive actions concurrently.

Solution(s)

1. We already have a (locks <path>) field for making actions mutually exclusive. We could easily upgrade this mechanism to a token with a finite number of available of resources. For example:

(resource
 (name foobar)
 (availability 10))

(rule
  (action 
   (with-resource foobar 2 (run ./foo.exe))) ;; this will consume 2 out of foobar. if 2 aren't available, it will wait until it is.
 ...)

2. We already have an existing "resource" - jobs. We could make this resource available more explicitly: (with-resource dune-jobs 3 ..) to ask for 3 slots out of -j rather than just one.

cc @snowleopard

The issue names no files, tests, or entry points. Start by reviewing the existing (locks <path>) mechanism and -j job handling; done would require choosing and specifying a resource model and its behavior before implementation.