Proposal: Changing Open and NewTransaction

[sio4]

Hi, all!
I want to try to make a change on connection.go. The goals are:

• To support database specific initialization
• To support database specific transaction handling

(It is indirectly related to #281)

Details

Current ideas are:

For Connection.Open()

Sometimes we need some initial settings for the connected database and sometimes we need to gather some information from the connected server. (such as protocol version, server version,...) I already added some codes for CockroachDB to check the version of engine so we can handle version differences. In my opinion, the best chance to get that information is when we connect to it very first time.

So I will try to add something like:

if err == nil {
    c.Store = &dB{db}
    c.Dialect.ConnectCallBack(c)
}

and with ConnectCallBack(), we can add some database specific codes to initial setup and/or to get some information.

For NewTransaction()

I think transaction handling should be moved to dialect completely since there are some differences on transaction handling.

func (c *Connection) NewTransaction() error {
	return c.Dialect.NewTransaction(c)
}

Especially for CockroachDB, I found that transaction handling code is all of there own driver. (I cannot sure for now we can use that code or not, anyway.) And sometimes, we need to do some action for specific database engine when we get a new session (individual connections) and the NewTransaction() is the best place to do it.

Please think about my proposal and give me your opinions!

Thanks