initial impl of Index Aliases

an IndexAlias allows you easily work with one logical Index
while changing the actual Index its pointing to behind the scenes
Changing which actual Index is backing an IndexAlias can be done
atomically so that your application smoothly transitions from
one Index to another.
A separate use of IndexAlias is allowed when the IndexAlias is
defined to point to multiple Indexes.  In this case only the
Search() operation is supported, but the Search will be run
on each of the underlying indexes in parallel, and the results
will be merged.