[Mono.Profiler.Log] Fix excessive byte array allocations.

[Mono.Profiler.Log] Fix excessive byte array allocations.

The problem was that the base Stream.ReadByte () method allocates a temporary
byte array and then calls Read (..., 1) on it. To solve this, we override
ReadByte () in LogStream and use a private buffer to hold the result.

It's a bit of a mystery to me why Stream.ReadByte () does it this way. The
documentation for Stream explicitly says that instance methods aren't thread
safe, so it would be perfectly fine for Stream.ReadByte () to do what we do
here...