... 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

in our loops.

is not thread-safe.

Switch from random(3) to testable VRND for same reason.

Fixes	#2684

Reported by:	ernestojpg@github

Fixes: #2700

For the purpose of our signal handler (and, in particular, the stack
overflow detection heuristic) they are identical.

Fixes #2695 (as confirmed by @lkarsten)

Ref: #2693

This is the case for the ctx created for director operations as first
introduced with 5536f102

Alternatively, we could always set ctx->handling to some location
with the same scope as the ctx, but as we are not inside VCL (and
just happen to use the same context for simplicity and reusability
of VRT/vmod functions), adding specific handling appears to be the
cleaner solution.

Fixes #2692

Properly shut down streams where we could not process a req
body (e.g. via a failing cache_req_body). The rxthread may be waiting on
this stream to consume a DATA frame - let it know before we go away to
avoid it being stuck forever.

If we have one or more streams set up because of a PRIORITY frame _and_
for some reason a tcp FIN is lost (or a client merely keeps a connection
open), this transaction would be stalled indefinitely.

Avoid failing sessions with pipelined data.