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>