ES-based Spouts use a static Client instance

[jnioche]

The ES clients are thread-safe, there is no reason why they should not be shared among the Spout instances within the same worker.

@mattburns what do you think?

[mattburns]

Sounds like a good idea to me.

For future reference, here's a good pattern when you need to synchronize access to the construction of a a shared object: https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom

Totally irrelevant in this case because it happens within the open method and so doesn't need to be optimised for millions of repeated invocations. I just thought it was interesting ;)