[Mono.Profiler.Log] Load entire buffers before processing them.

[Mono.Profiler.Log] Load entire buffers before processing them.

This should reduce the amount of I/O system calls we do, and should be fine
in terms of memory usage since the profiler tries to keep buffers small (64k),
and latency since the profiler flushes buffers every second.

This changes the API somewhat so that users no longer have to construct a
LogReader; this is now done internally, and the type is no longer public. Also,
the LogProcessor now leaves the given stream open, so it's up to the user to
close it.