Don't yield in oplog entry handler
Simplify crossbar by making it synchronous

All existing listener callbacks were already calling complete()
synchronously, so this should not be a functional change.

This allows us to also eliminate the callback from crossbar.fire().

This in turn allows us to eliminate the `proxy_write` in
Meteor.refresh. The only purpose of that write was to keep the current
write fence open until fire's async callback got called; now that fire
works synchronously, it's not necessary!  (This relies on the fact that
write fences never get armed while they are the current write fence,
which now has an assertion.)
Ensure crossbar callbacks not called after stop

Hopefully fixes #1767.