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.
is the best thing we can get here a string match?  Or is there an error code kicking around somewhere?  If not... sadness