Added LastStmtSent and use it to retry on errors if statement was not sent

Previously, a failed connection could be put back in a pool and when the
next query was attempted it would fail immediately trying to prepare the
query or reset the deadline. It wasn't clear if the Query or Exec call
could safely be retried since there was no way to know where it failed.

You can now call LastQuerySent and if it returns false then you're
guaranteed that the last call to Query(Ex)/Exec(Ex) didn't get far enough
to attempt to send the query. The call can be retried with a new
connection.

This is used in the stdlib to return a ErrBadConn if a network error
occurred and the statement was not attempted.

Fixes #427

query.go

@@ -368,6 +368,7 @@ type QueryExOptions struct {
 }
 
 func (c *Conn) QueryEx(ctx context.Context, sql string, options *QueryExOptions, args ...interface{}) (rows *Rows, err error) {
+	c.lastStmtSent = false
 	c.lastActivityTime = time.Now()
 	rows = c.getRows(sql, args)
 
@@ -395,6 +396,7 @@ func (c *Conn) QueryEx(ctx context.Context, sql string, options *QueryExOptions,
 	}
 
 	if (options == nil && c.config.PreferSimpleProtocol) || (options != nil && options.SimpleProtocol) {
+		c.lastStmtSent = true
 		err = c.sanitizeAndSendSimpleQuery(sql, args...)
 		if err != nil {
 			rows.fatal(err)
@@ -414,6 +416,7 @@ func (c *Conn) QueryEx(ctx context.Context, sql string, options *QueryExOptions,
 
 		buf = appendSync(buf)
 
+		c.lastStmtSent = true
 		n, err := c.conn.Write(buf)
 		if err != nil && fatalWriteErr(n, err) {
 			rows.fatal(err)
@@ -460,6 +463,7 @@ func (c *Conn) QueryEx(ctx context.Context, sql string, options *QueryExOptions,
 	rows.sql = ps.SQL
 	rows.fields = ps.FieldDescriptions
 
+	c.lastStmtSent = true
 	err = c.sendPreparedQuery(ps, args...)
 	if err != nil {
 		rows.fatal(err)