HiveServer2: Implement some admission control mechanism for graceful degradation when resources are exhausted

An example of where it is needed: it has been reported that when # of client connections is greater than hive.server2.thrift.max.worker.threads, HiveServer2 stops accepting new connections and ends up having to be restarted. This should be handled more gracefully by the server and the JDBC driver, so that the end user gets aware of the problem and can take appropriate steps (either close existing connections or bump of the config value or use multiple server instances with dynamic service discovery enabled). Similarly, we should also review the behaviour of background thread pool to have a well defined behavior on the the pool getting exhausted.

Ideally implementing some form of general admission control will be a better solution, so that we do not accept new work unless sufficient resources are available and display graceful degradation under overload.