Regression: safe read failing during client exit events

Dr. Memory's leak scan during the exit event is failing to handle safe_reads: it crashes instead, or on Mac64 it hangs. This is a regression.

I tracked it down to edb2eed6: for #2921 (closed), PR #3364 blocks all signals, not just asynchronous ones, before running exit code. This means that DR won't catch and report crashes, and more importantly clients can't use safe_read and other features in their exit events.