transaction Discard/Exec should put the connection back

[nightlyone]

Since the connection has been reserved for this purpose only and the Exec documentation even states that it puts the connection back to normal.

Requiring Close feels pretty cumbersome to me.

Please deprecate use of Transaction.Close and make it a no-op after Exec/Discard.

Normal and expected flow for transactions would be:

    t, _ := redis.Transaction()
    _ = transaction.Watch("mykey")
    // work and queue up commands
    if someError {
        _ = t.Discard()
    } else {
        _ = t.Exec()
    }

[xuyu]

A pull request will be appreciate.