Fix handling timeout errors

[mvijaykarthik]

The comment in database/sql/driver/driver.go mentions

// To prevent duplicate operations, ErrBadConn should NOT be returned
// if there's a possibility that the database server might have
// performed the operation. Even if the server sends back an error,
// you shouldn't return ErrBadConn.
var ErrBadConn = errors.New("driver: bad connection")

Network error could be a timeout error where the database might
have executed the query.

This commit adds a check so that ErrBadConn is not returned
for timeout errors

Fixes #422

[andreimatei]

Please reference #422 in the commit.
Haven't looked at the PR, but I believe the problem is more general than just timeouts.

[mvijaykarthik]

Referenced the issue in the commit. The original post in #422 is related to timeouts. I'm facing a similar issue where an INSERT query times out, and is internally retried by database/sql, and I get a duplicate primary key error for a row being inserted the first time (because the first insert succeeded in the database, but timed out).

[maddyblue]

I agree this should apply for more errors. I've opened #730 to address this.