MB-7656 Cleaner interface for btree lookup callback

We define the key argument as a void pointer but all the time
we pass in a sized_buf pointer, which is what all the callers
expect and makes sense. Therefore simplify this by changing the
interface to accept a sized_buf pointer instead of a void pointer.

Change-Id: I95c499baabf26b793588bebb965df88210132521
Reviewed-on: http://review.couchbase.org/29449
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>