Add some XXX comments about when we should re-poll
Fix "drop collection" test failure

There was a race condition in the manipulation of the write fence by the
drop collection code. Specifically, when seeing a "drop collection"
oplog entry, OplogObserveDriver took no immediate action and instead
deferred some "should I re-poll now or later?" logic.

This allowed the write fence's oplogHandle.waitUntilCaughtUp code to
think that everything was "caught up" even though the effects of the
"drop collection" entry had not yet affected the cursor's state (by
transitioning it away from the STEADY state).  The fix is to ensure that
the state-change aspects of processing the entry occur immediately; the
actual re-query is still deferred in order to not block the oplog tailer
from continuing.

Reported by: @awwx

(The previous failure can made consistently reproducible by replacing the
`Meteor.defer` that is removed by this commit with a
`Meteor.setTimeout(,200)`.)