Inlcude terms crashes on larger sites

The sitemap always runs into timeouts (more than 240 seconds) if a site has a lot of taxonomy terms. Additionally the sitemap file exceeds 10 MByte in size so it gets refused by Google.

This is independent of the actual value for GSITEMAP_CHUNK_SIZE, since all terms are written in every chunk file. Obviously, the terms should be splitted into chunks instead, too.