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.
yeah I think I will rewrite this to use mongoc-array for allocation since it does the power-of-two doubling already.