Btree Compare callback function - context parameter

It would be usefull if the BTree compare function allowed the
application developer to supply a 32 bit value to the setBtCompare
method.

This value could then be called passed back to the app developer in
the compare callback function. This would be to enable the app
developer to associate this with, for example, a C++ object which
could have information about the kind of keys stored. This would
facilitate the support of different kind of indexes.

I know that the extra overhead of this parameter will introduce a
performance penalty. However we can avoid this by introducing it as a
second setBtCompare function, or by replacing the current Db parameter
to the callback method with this 32 bit parameter instead.

Thoughts ?