Streamline configuration.h

Streamline configuration.h

Move more out of configuration.h into configuration_impl.h as
configuration.h is large and included in a number of places.

Change-Id: I099ab79253372675f70d677c13e73a2d9c59d74f
Reviewed-on: http://review.couchbase.org/76475
Reviewed-by: David Haikney <david.haikney@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>