Properly fix #14 and fix #24

[coagmano]

The previous fix for #14 (PR #27) had an issue where it could break tests that weren't affected by #14 before because the collections object returned from PublicationCollector will have the randomised names, where previously folks could just reference the original collection's name like so:

collector.collect('projects', collections => {
    assert.equal(collections.projects.length, 3);
    done();
});

Using the original collection name creates the issue that LocalCollectionDriver caches the contents of named collections. This PR pre-emptively removes the documents from the local collection when calling restore, so future stubbed collections won't have the old docs.

To handle the case where multiple null/unnamed collections are stubbed, this PR uses a Map to pair collections with their local collection stubs, solving #24.

[hwillson]

Make sense @coagmano, great fix - thanks very much!