Ensure that lang-go cache dir doesn't grow unbounded

[nicksnyder]

lang-go writes a cache to disk that is (in practice) a shared SSD on a node. lang-go needs to apply some eviction policy so that the cache doesn't grow unbounded.

see https://github.com/sourcegraph/sourcegraph/issues/627#issuecomment-461108211

[chrismwendt]

Sorry I didn't get to this sooner, hopping on it now.

[chrismwendt]

It looks like there's a file descriptor leak in go-langserver. Each time I trigger a new WebSocket connection, the zip archives get opened again and the output of lsof +D /tmp/lang-go-archive-cache/ keeps growing.

I think this is happening because the archives for dependencies get combined into a single VFS, but their lifespan is not tied to the combined VFS in any way (probably should be refcounted).

[chrismwendt]

Fix pending sourcegraph/go-langserver#359