That is, when it's a request or backend transaction, otherwise it's
still logged with VXID 0. The goal is to allow a correlation between
VCL statements and their workspace footprint by sharing the same
transaction instead of being split between raw and vxid scopes. That
kind of debugging becomes possible when the workspace debug flag is
used and VCL_trace records are not masked.

Given the verbosity of both workspace and trace logs, we would need
new tools to process transactions for this purpose and until we can
link everything we have a chicken-egg problem.

This change does not go all the way and relies on the thread-local
worker reference to opportunistically log into its VSL buffer when
it has one. When the workspace is initialized, the buffer may not
be ready yet, so some logs may still end up raw. What's likely going
to be missing is workspace early initialization and teardown: in other
words before and after VCL execution, which should be good enough for
the purpose of debugging workspace operations in VCL tasks.