Add hashing of req.body, bereq.body and resp.body to .update()

See merge request uplex/varnish/libvmod-blobdigest!1

.update() can now hash all bodies except beresp.body, which is
more difficult because it would prevent streaming.

The msg argument is now optional in order to allow hashing of only
a body.

This is not yet particularly apparent with the current error conditions,
but in follow up commits we are going to make use of this facility for
good causes:

In some circumstances, it might be desirable to explicitly handle
error conditions - and in fact the .update() method is already,
historically, prepared for this pattern from the days before intro-
duction of VRT_fail()

in the next commit, we are going to add the option to not fail

Remove duplicate call to update() using a local variable for the hash ctx.

This also adds a check for finalization from vcl_init/vcl_fini

The other methods/functions name it msg

Ref:
https://github.com/varnishcache/varnish-cache/commit/681c11998ec56dc4460a30585210f95a7850ea3b

Ref: https://github.com/varnishcache/varnish-cache/commit/5450550906d0809dfc21ae9d3a5b14446c1809d4

Ref:    d67993451bda7eeb7c0ea3c5528a8353f283a190

This reverts commit 5829411d.

ref: 0c74d1b99f1f43fc99725fe14322760afa7858ee

Ref: e8985f22711fe9a342b27a10052ae3655f64435c

ref https://github.com/varnishcache/varnish-cache/issues/2840

The reason is the same as for
https://github.com/varnishcache/varnish-cache/issues/2708 :

VRT_priv_task may now return NULL for out-of-workspace errors, see
https://github.com/varnishcache/varnish-cache/commit/de2b431086a13b243dc8b3e71cd8697db1df2c7f