Expose DialStrategy function to user for custom connection routing

[jkaflik]

Expose a way to have a customized way how connections created: (the result of dial function)

opts.DialStrategy = func(ctx context.Context, connID int, opts *clickhouse.Options, dial clickhouse.Dial) (clickhouse.DialResult, error) {
	return dial(ctx, "server.host:5678", opts)
}

Default DialStrategy has following implementation clickhouse.DefaultDialStrategy:

func DefaultDialStrategy(ctx context.Context, connID int, opt *Options, dial Dial) (r DialResult, err error) {
	for i := range opt.Addr {
		var num int
		switch opt.ConnOpenStrategy {
		case ConnOpenInOrder:
			num = i
		case ConnOpenRoundRobin:
			num = (int(connID) + i) % len(opt.Addr)
		}

		if r, err = dial(ctx, opt.Addr[num], opt); err == nil {
			return r, nil
		}
	}

	if err == nil {
		err = ErrAcquireConnNoAddress
	}

	return r, err
}

clickhouse.DialStrategy function receives a clickhouse.Dial function that needs to be triggered by the user if a new connection is needed. It returns a dedicated DialResult that wraps over internal and coupled connect struct.
It's a hacky solution, as connection and pool management are highly coupled in the library. Until we find a better design in v3, we need to have a trade-off.

Resolves #772