Re-apply 'primary preferred' read mode
Re-apply 'primary preferred' read mode

Removed in https://github.com/alphagov/content-store/commit/e7383e24e4dd6a733f04538659ac379c9151e523
the default read mode for mongodb is primary:
'By default, an application directs its read operations to the primary member in a replica set.'
However in a scenario where the primary fails we should be
able to read from the secondaries until the primary is healthy.
This would allow frontend applications to respond with albeit
slightly stale data rather than 500s.

alphagov/content-store,alphagov/content-store