Reduce unnecessary Mongo polling.

Specifically, updates and removes that specify a specific _id, and all inserts,
should not require live queries that specify a specific different _id to poll.

This involves changing the InvalidationCrossbar matching semantics. I suspect
that a future version of the InvalidationCrossbar may want the old semantics, if
the notifications it is receiving are more oplog-driven than command-driven, but
this will work for now.

Also, make the O(1) optimization on minimongo remove("id") actually work.
Previously it was declawed by the fact that Meteor.Collection.remove normalizes
to {_id: "id"}.