get = async (key) => {
    for await (const doc of iterator()) {
      if (key === doc.key) {
        return doc
      }
    }
  }
Gets a document from the store by key.
@function
@param {string} key The key of the doc to get.
@return {Object} The doc corresponding to key or null.
@memberof module:Databases.Databases-Documents
@instance