MB-8910 Fix occasional group cleanup failure with JS views

JS reduce contextes must exist when computing the new bitmaps (final
step of cleanup) after the cleanup itself finishes.

Stack trace:

[couchdb:error,2013-07-15T11:54:49.405,ns_1@10.13.5.112:<0.1022.0>:couch_log:error:42]Set view `default`, replica group `_design/dap`, cleanup process <0.18694.4> died with unexp
                                                                                                           [{mapreduce,
                                                                                                             reduce,
                                                                                                             [undefined,
                                                                                                              []]},
                                                                                                            {couch_set_view_mapreduce,
                                                                                                             reduce,
                                                                                                             2},
                                                                                                            {couch_set_view_group,
                                                                                                             '-init_group/3-fun-3-',
                                                                                                             7},
                                                                                                            {couch_btree,
                                                                                                             full_reduce,
                                                                                                             1},
                                                                                                            {couch_set_view_util,
                                                                                                             '-compute_indexed_bitmap/2-fun-0-',
                                                                                                             2},
                                                                                                            {lists,
                                                                                                             foldl,
                                                                                                             3},
                                                                                                            {couch_set_view_util,
                                                                                                             cleanup_group,
                                                                                                             1},
                                                                                                            {couch_set_view_group,
                                                                                                             cleaner,
                                                                                                             1}]}

Change-Id: I6489ff7bbb383e9c5e5898756076df76b927d6f6
Reviewed-on: http://review.couchbase.org/28443
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>