MB-7656 Make btree lookup callback arguments const

Because they're not supposed to be modified nor does it make sense
to. Adding const enables logical error checks at compile time.

Change-Id: I43a5146c9807a01c599621a9be9fea6c7cf43323
Reviewed-on: http://review.couchbase.org/29450
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>