Catch ESRejectedExecutionException on node close

When a node shuts down thread pools might throw
ESRejectedExecutionException and our test framework fails tests if
exceptions are not caught hitting uncaught exception handler.
Rename IndexShard#searcher() to #acquireSearcher()

Based on recent bugs ( #3652 ) where searchers were acquired multiple times
but never released 'IndexShard#searcher()' has not a more accurate name.

Closes #3653