[TEST] run REST tests against multiple nodes (round-robin)

Multiple nodes are now started when running REST tests against the `TestCluster` (default randomized settings are now used instead of the hardcoded `1`)

Added also randomized round-robin based on all available nodes, and ability to provide multiple addresses when running tests against an external cluster to have the same behaviour
Log warning in case of double release of the IndexSearcher instead of throwing an exception.