Improve ErrBadConn

[methane]

We have dropped old go versions. So we can improve how ErrBadConn handled.

Since database/sql uses IsValid() and ResetSession() to check connection, we can return i/o errors without translating them into ErrBadConn.

We should remove errBadConnNoWrite too.

[methane]

fixed by #1641 and #1642.