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
somewhat implement write fence (latency compensation)
xxx now
more progress
ok, rewrite is done.
now "replace" updates work too. also give up on idea of knowing
what fields changed (more correct, does require an in memory diff).

many tests pass.
Until I do the right thing, it's slightly more right to do the query before
listening.
handle _id properly in replace.  many tests pass.
all tests now pass (mostly because I made sure the failing tests
avoid oplog, but they are testing basically implementation details)

there's a "insert found for already-existing ID" somewhere though

trasnform: null, interesting...
oops, stop listenersHandle
rename writes collection to sequencers. will be used for observe catchup too
doc fetcher!