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>
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>
disable visibility

Change-Id: I48d325712a252d9d9d7fd5437b8086d98f3138c2
Reviewed-on: http://review.couchbase.org/29497
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Dustin Sallings <dustin@spy.net>
Python: Header position should be 64-bits long

Fixes MB-7347

Change-Id: Ibe6bca62d453405eb2e7a38ef2514a3c51d1732d
Reviewed-on: http://review.couchbase.org/29590
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>