Create iterators in order

[vweevers]

This fixes the (currently disabled) create-stream-vs-put-racecondition.js test in levelup (Level/levelup#497). That test was originally meant for leveldown I believe, before it had snapshots. Today, it's more relevant for memdown, which is secretly synchronous. E.g. it does (simplified):

memdown.prototype.put = function (.., callback) {
  this.store.put(..)
  process.nextTick(callback)
}

Rather than:

memdown.prototype.put = function (.., callback) {
  process.nextTick(function () {
    this.store.put(..)
    callback()
  })
}

Before this PR, if you did the following on a deferred db:

db.iterator()
db.put(..)

Then on the underlying db it would actually be executed as if you did:

db.put(..)
db.iterator()

Which in the case of memdown, because it is synchronous, means the put was written before the iterator (snapshot) is created. Does it matter? Probably not a whole lot, but I think this PR makes for a less surprising behavior.

[vweevers]

Ah, damnit, this would affect reachdown 😄 (currently detects deferred-leveldown by presence of _operations and _iterators)

[vweevers]

In the interest of moving on, I'll restore this._iterators = [], with a comment saying it's temporary.

Edit: or, adding type (Level/community#82) might do the trick.