SmtpSessionManager lock

[tinohager]

I think it would make sense to replace the lock logic with a semaphore.

https://github.com/cosullivan/SmtpServer/blob/cffd7df707cd0108e816b26659b277cb15a35db9/Src/SmtpServer/SmtpSessionManager.cs#L81

https://medium.com/@tyschenk20/mixing-traditional-locks-with-async-code-in-c-27431f857e01