async changeUsername (uid, newUsername) {
    if (!await this.checkUsername(newUsername)) return -1
    const result = await db('users').where('id', uid).update({
      username: newUsername,
      lastSeen: (new Date()).getTime()
    })

    return result
  }
Change user's username

@param {Int} uid user id
@param {String} newUsername new username

@returns {Int}