The goal of the workspace emulator is to replicate the regular workspace
behavior with individual allocations and make it work transparently.

It's the successor of the workspace sanitizer from #3320 with notable
differences:

- enabled at configure time instead of run time
- in a separate source file instead of mixed in
- using sparse allocations instead of built-in red zones

This means that the workspace emulator can be combined with regular
sanitizer, in particular asan and lsan. If available, asan's public
interface is used to mitigate the possible overflow of a reservation
after some of it was released.

Even without sanitizers, the fact that we integrate with jemalloc by
default and enable its abort and junk options in varnishtest is enough
to detect a use-after-free in some cases.

With sanitizers though, the workspace emulator can observe #3550.

One drawback is that the logic is split in two files, and some functions
are identical in the two files. It might be possible to split cache_ws.c
into something like cache_ws_alloc.c and cache_ws_util.c for example.

Closes #3320
Refs #3550
Refs #3600