Improve actors' budgets

[loyd]

Now we're limiting the number of recv() calls to avoid starving other actors on the same scheduler's thread.

However, there are several ways to improve it:

• Reset the counter if the mailbox is empty. Requires moving to try_recv() + separate recv().await calls.
• (Need to discuss it). If the current timestamp is calculated already (e.g. for metrics), check the elapsed time instead of just counter.