MB-8020 Change compacted file layout

Compactor will now output all doc bodies, then the Sequence b-tree, then
the ID b-tree, rather than outputting the doc bodies and the sequence
b-tree interleaved.

Change-Id: I0cde3764a1f23c0e8547f27e12da670715928125
Reviewed-on: http://review.couchbase.org/25604
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>