Added write rate-limiting.

Also changed 'doc' to 'docs' in find, since the general case is multiple documents.
Fixed a few minor issues.

 - Switched the event registration for `initialize` on collections to `once`, not `on`.
 - Fixed a race condition where multiple instances of a collection could be created before the first collection finished initializing.
 - Fixed double-call of a callback in the event of an error condition in find.