Thread-safe stringToNode() / pg_strtok()

Hi,

In line with various other efforts making things more re-entrant and
thread-safe, here's a patch that makes stringToNode (and pg_strtok
with it) thread safe.

There are other things we could do to improve this code (e.g.
de/serializaing based on node descriptors instead of generated
functions), but I consider such improvements to be out of scope for
this patch; the attached is just a simple move from global variables
to a context struct passed down the call stack.

Kind regards,

Matthias van de Meent
Databricks (https://www.databricks.com)