[sgen] Refactor collection logging

[sgen] Refactor collection logging

We had the problem that we were logging at sweep finish which can happen anytime. If we happen to suspend a thread, while doing a GC, in the middle of logging we might deadlock if we wait for sweep to finish (which can be blocked waiting for logging).

When logging multiple collections in the same stw pause, we would not log the real results for both collections since we would compute the major sizes at the very end.

We logged collections in sgen_perform_collection, outside the function calls for specific collections, which makes it cumbersome to add additional log entries.

This refactoring solves these issues by having GC code log info at any time. This log entry is enqueued in a list of pending entries. We flush the list of logs, as before, at the end of the stw, when the world is restarted.