Add support for stopping native index builder

Similar to the native compacter and native btree updater,
added ability to send a stop message to the index builder
and wait for native process to exit.

Change-Id: Ibda0ef728100266508407f6512f6f12dac2080d3
Reviewed-on: http://review.couchbase.org/32549
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>