Avoid regression in the size of XML input that we will accept.

Avoid regression in the size of XML input that we will accept.

This mostly reverts commit 6082b3d5d, "Use xmlParseInNodeContext
not xmlParseBalancedChunkMemory".  It turns out that
xmlParseInNodeContext will reject text chunks exceeding 10MB, while
(in most libxml2 versions) xmlParseBalancedChunkMemory will not.
The bleeding-edge libxml2 bug that we needed to work around a year
ago is presumably no longer a factor, and the argument that
xmlParseBalancedChunkMemory is semi-deprecated is not enough to
justify a functionality regression.  Hence, go back to doing it
the old way.

Reported-by: Michael Paquier <[email protected]>
Author: Michael Paquier <[email protected]>
Co-authored-by: Erik Wienhold <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 13