Improve handling of the HTTP body

[william-stacken]

The HTTP body is stored in HttpMessage.cs as a byte array, which is not ideal since it increases memory usage when transfering a lot of data. Maybe this could be solved by optionally setting a filepath or a stream that the body can be read from or written to instead?

This would make it possible to remove the ReadLog method from Logger.cs and replace it with a "WriteLogToStream" method instead, improving memory usage.

[william-stacken]

Now that there is a LogStream class and the pointless mutex in Logger.cs has been removed, it should be fairly simple to replace ReadLog with GetLogStream. One could then set the body in HttpMessage to this stream.