Sets the new engine before closing the previous one in IndexShard.resetEngine

[tlrx]

Current code closes the previous engine instance, creates the new engine and then updates the currentEngineReference:

    IOUtils.close(currentEngine);
    var newEngine = createEngine(engineConfig);
    currentEngineReference.set(newEngine);

This leaves more room for callers of getEngineOrNull() to pick a closed instance. Instead we can create the new engine first and atomically update the reference to the new instance.

[elasticsearchmachine]

Pinging @elastic/es-distributed-indexing (Team:Distributed Indexing)

[fcofdez]

LGTM. Thanks for fixing this

[kingherc]

Nice one. Just one comment before approving.

[arteam]

LGTM

[tlrx]

Thanks all!