Better support for large collections

Should address #195.

Two things that should help large collections:
1. When trying to infer schema, we no longer scan every single document in new collections, limiting ourselves to 100 documents (TODO: actually random sample - mongo makes this non-trivial)
2. use `_suppress_initial` so `observe.added` doesn't fire on every single document already in the collection (see https://github.com/meteor/meteor/issues/633#issuecomment-12716687)