add rewind function to QueryCursor

[passabilities]

Summary

Wraps the mongodb.FindCursor.rewind function to reset the cursor to its uninitialized state, effectively resetting the cursor to the beginning.

Examples

const options = {}
ModelClass.find(options) 
// - instance1
// - instance2
// - instance3

// ================================================

const cursor = ModelClass.findOne(options).cursor()
await cursor.next() // instance1
await cursor.next() // instance2

cursor.rewind()
await cursor.next() // instance1

[vkarpov15]

Wrap this in _waitForCursor(), because this.cursor can be undefined until async pre hooks run. Look at addCursorFlag for an example

[vkarpov15]

LGTM, merging this into 6.8 branch