SNOW-165479: Blocking thread on retry handler

[pcchannz]

https://github.com/snowflakedb/snowflake-connector-net/blob/3adccf292e8e582a9c56cbbfb039fba7a826807c/Snowflake.Data/Core/HttpUtil.cs#L207

Replace with asynchronous wait: await Task.Delay(backOffInSec * 1000);

I observed that during heavy load, the waiting time on retry handler could add up.

Prior updating the line, 35% of my application threads were spent on synchronous sleep.

After updating the line, it was around 8.5%

Thank you!