Add TxOptions to txn.Begin

[dnephin]

This will allow us to start read only transactions, or transactions with different isolation levels.

We're not using that yet, but after #3302 merges we'll be able to use it for handlers. An example of that can be seen here #3328.

We will also need this for long-polling. We need to use an isolation level that ensures that multiple queries in the same transaction all read the same snapshot (repeatable read).

[ssoroka]

could consider using variadic arguments to allow the opts to be optional.

Suggested change

	func (d *DB) Begin(ctx context.Context, opts *sql.TxOptions) (*Transaction, error) {
	func (d *DB) Begin(ctx context.Context, opts ...sql.TxOptions) (*Transaction, error) {

[dnephin]

I see gorm uses this pattern, but I consider this dangerous and I don't think it should ever be used.

varargs are great if you are going to use all the args, but using varargs for a single optional argument and ignoring the rest is going to cause confusion for anyone reading the code. Adding a nil for the parameter isn't really a big inconvenience. We could also follow the stdlib and have two functions for Begin, but at this time it didn't seem necessary since it was easy enough to add the nils.

[ssoroka]

Well, it's just a suggestion. The nil argument is a bit ugly, especially when that's the default case. From the project I've seen, this is a really common pattern and familiar to many Go devs.