MB-10138: Writing a header now also returns the position

The function to write a header now also returns the position. This change
was introduce in commit /79038773b2ed4beaa1e1bf2e889fcff4141ff49e [1][2].

This fixes the testrunner spatialquery.conf tests (tested on single
and 4 nodes).

[1] https://github.com/couchbase/couchdb/commit/79038773b2ed4beaa1e1bf2e889fcff4141ff49e
[2] http://review.couchbase.org/28816

Change-Id: Ied2f89ff72990e1f93e5eb7cbe91338684e9e06f
Reviewed-on: http://review.couchbase.org/33284
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>