add DropAllIndexes() method

[feliixx]

Create a new method to drop all the indexes of a collection in a single call.

Currently, the documentation says ( in Indexes ):

For example, this snippet would drop all available indexes:

indexes, err := collection.Indexes()
if err != nil {
    return err
}
for _, index := range indexes {
    err = collection.DropIndex(index.Key...)
    if err != nil {
        return err
    }
}

But this code fails as it tries to delete all indexes, including index on _id, which can't be deleted.

The correct way to achieve this is :

err := db.Run(bson.D{{"dropIndexes", c.Name}, {"index", "*"}}, &result)

This PR remove the code sample from Indexes documentation, and provide a new method DropIndexes() which is a wrapper around above command

[domodwyer]

Hi @feliixx

Thanks for the PR! Really appreciate you taking the time to contribute your changes upstream.

Would you mind changing DropIndexes() to DropAllIndexes()? - the former is a little too close to the singular DropIndex() with very different results - keeps code reviews easy :)

Thanks again!

[domodwyer]

Rename DropIndexes to DropAllIndexes for clarity.

[feliixx]

Name updated. Thanks for the review !

[domodwyer]

Thanks a lot @feliixx - it's really appreciated.