This replaces the sources in the tree verbatim with upstream version
3.13.4. It does not work with GPDB as it is, but I wanted to have the GPDB
changes in separate commit. The next commit will fix it so that it works
again.

The reason to update it now is that we are working with mergign PostgreSQL
v12 into GPDB, and the old 3.7 version we had in the repository will not
work with PostgreSQL v12. This version should work with both PostgreSQL
9.6 where we are at the moment, and with v12.