Nhat Nguyen f2e8b17696 Do not create engine under IndexShard#mutex (#45263)
Today we create new engines under IndexShard#mutex. This is not ideal
because it can block the cluster state updates which also execute under
the same mutex. We can avoid this problem by creating new engines under
a separate mutex.

Closes #43699
2019-08-26 17:18:29 -04:00
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