Going around on:fork: to debug announcement

[StevenCostiou]

This should not be considered for merging yet, but for review.

We're trying to make configurable the debugging of announcement, that use on:fork: to make unhandled errors non-blocking when happening during an announcement delivery.

The problem in case of bugs is that we lose the stack that led to the unhandled error, and we're left with few information.

Here we use a metalink to replace the on:fork: with an on:do: and see the full stack.
We now have the stack back, it becomes blocking again, but can be (de)activated at will.

It works, but lots of questions:

• at which level should that be? In Pharo or in the tools?
• what other solutions than metalinks do we have? ==> the main advantage is that we're not modifying the on:fork: directly and we then do not impact the system for users not interested by such debugging feature