Update field name in GenericSearchProvider to reflect changes
from nasa/openmctweb#193. Avoids exceptions on mutation.
Additionally, add test case exercising relevant code and verifying
that reindexing is scheduled upon mutation as expected.
The SearchAggregator exposes it's constants to add stability to
tests.
It also has a fudge factor which increaases the number of results
it requests from providers to better support pagination when using
client side filtering.
Updated the generic provider test. Removed index
checking from the generic search worker, because
that checking is already done (more efficiently)
in the generic search provider.
Moved the elasticsearch provider to
platform/persistence/elastic. Then moved
the search aggregator and the generic
provider into a services folder within
the search folder.
