Allow darklaunch control over search index queue operations

This patch wires darklaunch data received from request headers into
the chef_object_db where all interaction with the search index queue
occurs. This sets us up to add feature flipped behavior for testing
new search infrastrucutre. For example, we can add code to do in-line
flatten/expand and POST to a solr4 instance while continuing to put
items on the current search index queue.

Two new xdarklaunch flags are introduced:

send_to_solr4 - this flag enables currently stubbed code for taking an
action that coincides with putting an add or delete on the queue for
indexing.

disable_send_to_rabbit - if set to true, this flag prevents the code
from sending messages to the rabbit based search indexing queue. We
would use this as part of a migration strategy once an org is on the
new search infrastructure.
Add chef_object_db:bulk_delete_from_solr/4

Exporting this new helper makes it easier to share code with EC and
encapsulates the darklaunch behavior around adding/removing items from
the solr index.