MB-7629, MB-7656 Add mapreduce library

To be used by the view compactor, view builder and view updater.
This is a C++ library, that links against the V8 JavaScript engine,
but it exposes a pure C API to the outside.

Change-Id: I4f80f7d490e31dd3b43e50007a6c23133baaa7b5
Reviewed-on: http://review.couchbase.org/25257
Tested-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>