[TEST] Expect all shards to fail if full refresh failed
Throw EngineClosedException if engine is closed

Currently we throw misleading exception in acquireSearcher
if we try to acquire while we are failing the engine. We should
throw an EngineClosedException instead.

Relates to #5633
Enrich exception message if searcher can not be aquired

This commit also moves the log message after the ensureOpen() call
to only log an error if it's really an erro condition.
[TEST] Add ThreadLeaks protection to ElasticsearchPostingsFormatTest since we keep running clusters around in the JVM
[TEST] Removed last occurences of cluster_seed, no longer used

Relates to #5233
[TEST] Add ability to start nodes in async mode in TestCluster
this will help speed up when starting multiple nodes in a test
closes #5644