Fix Event-Exception: "Microsoft.Data.SqlClient.WriteCommandBefore"

[AndrewP-GH]

Fix #86: I found that the reason for the exception was the race condition in PropertyFetcher when we store fetcher in the static field.

What do you think of this solution?

P.S. I've also replaced Activator.CreateInstance with Expression.New for better performance

[AndrewP-GH]

@cwe1ss