MB-10044: Partition UUID is an unsigned 64-bit integer

The partition UUID is handles as unsigned 64-bit integer,
we should do the same on the Erlang side. This change doesn't
really change the functionality as over the wire it gets
encoded as a binary anyway.

Change-Id: I10a79f9810f0c352df58936e393f30e5ca8d1f70
Reviewed-on: http://review.couchbase.org/32917
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>