Serialize $out to old format if featureCompatibilityVersion is "4.0"

By "old format", I mean the format described here, where only a string with the name of the output collection is specified. This allows a binary version 4.2 mongod to play nicely with binary version 4.0 mongod servers in a mixed-version cluster. If any 4.2-only options are specified, an error should be thrown.

As part of this ticket, we'll write tests for $out in the multiversion suite.