Performance of SearchService bridge to Core Search

When doing searches, the CoreSearchBackend that bridges the SearchService to the CoreSession needs to allocate a session as none is passed currently.

This session must then be closed (to avoid leaks), but the returned documents are then unusable (or if they are used, they'll recreate a connection to the session unexpectedly when the caller may wish to use another connection).

It must also be able to do proper batching.