MB-7656 Add view reducer context object declaration

This is the object used to pass extra data to view reduce
(and rereduce) functions and to allow these functions to
return extra information too to the caller, such as view
reduce specific errors and later errors from the JavaScript
engine.

Change-Id: I9da5da8cc392282353004e6a8b2f0aedc9d0186a
Reviewed-on: http://review.couchbase.org/26671
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>