LogStash should call shutdown hooks

Currently, LogStash will shutdown by waiting for events to flush out and then call `do_close` on plugins. If `--allow-unsafe-shutdown` is specified then LogStash shutdown watcher will call `Kernel.exit` after some time.

However, ZeroMQ plugins will block under certain condition (this is by design of ZeroMQ), thus events will never flush out and `do_close` will never be called. The way to unblock the sockets is by terminating the ZeroMQ context. This can be done allowing a shutdown hook to be called before exiting forcefully.