Allow setting connection max lifetime

[chrisgilmerproj]

I'd like to be able to set the maximum lifetime on a DB connection. My use case is that I'm authenticating to the DB using AWS IAM roles which have tokens that expire every 15 minutes. So my connection shouldn't last longer that that time before I need to generate a new token and a new connection.

I am writing this PR because I can't seem to do this and it seemed easier to contribute back:

if db, ok := connection.Store.(*pop.dB); ok {
  sqlxDB := db.DB
  sqlxDB.SetConnMaxLifetime(10 * time.Minute)
}

[stanislas-m]

Thanks!