MT and WP Importers memory usage 2.3 fixes

[ryan]

Most of the importers slurp entire files into memory and store the results of parsing into memory, making for a big footprint. They also use lots of unnecessary regular expressions. Importers should parse files one line at a time and write posts to the DB as they are processed to help keep us within memory limits.

Reworked MT importer

(In [5684]) MT importer rework. see #4421

import-memory-reduction-r6606.patch reduces memory usage in the WP importer by avoiding the use of file(). It still doesn't process entries one by one, but it's a start.

(In [5802]) Don't slurp in entire file. Props tellyworth. see #4421