Implement feedback for when a GC happen

[danielfireman]

There are quite a few runtimes which developers can not disable the GC. Even though the proxy is conservative when setting the heap consumption limits, that can still leave room for mistakes. In those cases, the upper limit must decrease.

That needs to come from a mechanism which tells the proxy that one or more spurious GC had happened. Instead of checking every answer, we could add another field from the memory check message. That could return the number of spurious GCs that happened since the last check.