CDRIVER-425 use listCollections to enumerate collections.

I'd be surprised if I wasn't leaking memory somewhere. Will valgrind this tomorrow - but in the meantime I would appreciate style & design comments, etc.

Tested against 2.7.7 and 2.6.3.
If this goes in after the libbson copy_to_excluding_noinit change that came with a deprecation, you may want to use that function instead