Make RegisterDialect concurrency safe

[amanangira]

Describe the bug
In our application we are registering a custom dialect for Snowflake and returning the newly registered dialect for all the queries. Recently we added a snippet that would launch multiple goroutines utilising the custom registered dialect. However, this caused a concurrent map read and map write error. On debugging this I realised that the RegisterDialect doesn't have any kind of mechanism to handle concurrent read or writes.

To Reproduce

func Dialect() goqu.DialectWrapper {
	opts := goqu.DefaultDialectOptions()
         // configure desired options
	goqu.RegisterDialect("snowflake", opts)

	return goqu.Dialect("snowflake")
}

// And concurrently invoke this Dialect() method. 

I understand the example provided isn't full blown example but should help understand the underlying problem I am talking about. I did solve this problem in my case by wrapping the the RegisterDialect under a sync.Once for myself.

I also want to highlight just the minimum example isn't enough to reproduce the scenario. And I cannot share my code snippet without exposing the IP.