Sibling index: On merge conflict, add index if asked to do so.

This is for providing an easy, cheap and reliable way to query for conflict-burdened keys.

The mechanism is: on merge conflicts (i.e. whenever the result of a merge has >1 contents), if user-metadata "X-Riak-Meta-X-Index-Siblings" has a value of "true" (in any of the contents), then add to the resulting object an index item "x_riak_siblings_int: <number of siblings>" (and remove any old such indexing).

The two special names are rather arbitrary; please change them if you find better ones.

(This patch was mentioned on riak-users: subject "Indexing conflicts (feature)")