Compilation server cache size

[ncannasse]

Usually there should be no need to clear the cache of the compilation server. However in case the user switch between many different projects and/or different defines (causing different cache contexts to be created) this could trigger a lot of memory to be allocated and never released.
#5735 is one particular example of that

What should be done would be for each cached module / context store a "LastUsed" timestamp, then when we grow to a certain threshold of memory (measurable with Gc.quick_stat and to be configurable with a define) we could discard the contexts and modules that have not been used for a long time.

For the modules it's a bit more complex since it requires to discard as well all the dependencies TO it (recursively).

[ncannasse]

With 64 bits this is no much longer an issue, closing for now.