Add a debug helper for devel like the one for SQL

It would be convenient to have a way to log queries passed to MongoDB during a page cycle, much like the query trace offered by Devel module on \Database.

Boris (drush core-cli) shows query cursors quite nicely, like this:

$q = $c->find(['pid' => ['$gt' => 5]], [_id => 0, started_iterating => 0, dead => 0])
  ->sort(['pid' => -1])
  ->skip(5)
  ->limit(10);

MongoCursor {#294
  ns: "ninja.url_alias"
  limit: 10
  batchSize: 0
  skip: 5
  flags: 0
  query: array:2 [
    "$query" => array:1 [
      "pid" => array:1 [
        "$gt" => 5
      ]
    ]
    "$orderby" => array:1 [
      "pid" => -1
    ]
  ]
  fields: array:3 [
    "_id" => 0
    "started_iterating" => 0
    "dead" => 0
  ]
  started_iterating: false
  dead: false
}