MB-9730 Integrate native viewgroup cleanup support

An external native program `couch_view_group_cleanup`
is invoked to perform viewgroup cleanup. On successful cleanup,
viewgroup couch_file processes are refreshed to use new views
after cleanup.

A comparison of time taken for cleanup before and after this change
benchmarked by running `test/11-updates-cleanup-many-views.t` with
6400000 documents are as follows:

Before:
Removed 30000 values from the index in 4.468 seconds

After:
Removed 30000 values from the index in 2.364 seconds

Change-Id: I1ef010a5c1dad8366e90dbbba4bf93117a92377f
Reviewed-on: http://review.couchbase.org/30787
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>