Don't flush debug stream if debugging is disabled.
Thanks to x190.
Add const to parameters of DHT callback.

This is an incompatible API change.
Consult local storage when performing a search.

The likelihood of this triggerring in a fully grown DHT is vanishingly
small, but people are running modified versions of this code in
private DHTs.
Fix typo: incorrect constant in callback.

Thanks to Moritz Warning.
Add debug statement when finding local data.
Update CHANGES for 0.22.
Use _WIN32 instead of WIN32 to check for Windows

MinGW defines both _WIN32 and WIN32 (and may even be the only compiler
doing so). Microsoft and Intel compilers only define _WIN32. Use the
common one to eliminate the need in defining WIN32 explicitly.