MFB: More optimizations - less MM calls

MFB: More optimizations - less MM calls
Clearly separated fetching (physical reading) from decoding phases (data
interpretation). Threaded fetching added but disabled as needs more work for
Windows. For Linux needs some touches to add pthreads if this is enabled,
probably with a compile-time switch.
The code reorganisation makes it easy to add also async API, similar to
cURL's one.