go-sql-driver / mysql / 1146

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fewer driver.ErrBadConn to prevent repeated queries (#302)

According to the database/sql/driver documentation, ErrBadConn should only
be used when the database was not affected. The driver restarts the same
query on a different connection, then.
The mysql driver did not follow this advice, so queries were repeated if
ErrBadConn is returned but a query succeeded.

This is fixed by changing most ErrBadConn errors to ErrInvalidConn.

The only valid returns of ErrBadConn are at the beginning of a database
interaction when no data was sent to the database yet.

Those valid cases are located the following funcs before attempting to write
to the network or if 0 bytes were written:

* Begin
* BeginTx
* Exec
* ExecContext
* Prepare
* PrepareContext
* Query
* QueryContext

Commit and Rollback could arguably also be on that list, but are left out as
some engines like MyISAM are not supporting transactions.

Tests in b/packets_test.go were changed because they simulate a read not
preceded by a write to the db. This cannot happen as the client has to send
the query first.

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