... and introduce request functions for this purpose (for busy
objects, there is only one use case yet, so we don't).

Before we reset the workspace, we must ensure that there are no active
references to objects on it. As PRIV_TASK and PRIV_TOP have the same
lifetime as the respective workspace, they need to be destroyed. vmods
must not use workspaces for storing information referenced via any of
the other PRIVs unless the rollback case is considered.

Note that while this bug was exposed by
beeaa19c, it existed all along for any
vmod priv state stored on the workspace, so if a vmod happened to
access a TASK_PRIV stored on the workspace, it would likely have
triggered a magic check assertion as well.

I got plans for making std.rollback() more useful. While this change
is required to do so, it only partly covers the planned changes.

Fixes #2706