Add cursorSupportedByOplogTailing.
In "meteor run", run mongo in repl-set mode, so that oplog is available.

Note: this is pretty slow. Before merging this branch, should cache the fact
that replset has been initiated on port N so that on the next run, if the port
hasn't changed, we don't need to re-initiate.
Factor tailing code out of observe-changes code.
Don't reset the replset unless the port changes.

(Resetting the replset is slow!)
oplog checkpoint from friday
checkpoint monday morning
remove processing works.
very basic insert support
very basic update support
fix $unset