HSEARCH-1723 Allow testing helper SearchFactoryHolder to start multiple cluster nodes

Also allow more time in testhelper Conditions for slower machines
and reviewing default scale settings for BackendStressTest
HSEARCH-1726 Allow performance validation tests to run on MariaDB
HSEARCH-1725 Improve allocation rate of the new Lucene synchronous batching backend
HSEARCH-1727 Restore original package name for SearchFactory, break inheritance link with SearchFactoryIntegrator
HSEARCH-1727 Review API structure of SearchFactory: which methods should be exposed on API vs SPI vs hidden as implementation
HSEARCH-1727 Hide some core implementation types
HSEARCH-1727 Rename SearchFactoryIntegrator to SearchIntegrator