in_kubernetes_events: fix k8s events end of stream deadlock

[ryanohnemus]

The http client connection used to watch (stream) k8s events from the kube api server could deadlock the kubernetes_events plugin if the k8s api server ends the connection with a standard end of chunked stream instead of a different connection closure that would result in the http client returning an FLB_HTTP_ERROR. This same deadlock can also occur if there is a http client connection initialization error.

In either scenario the ctx->streaming_client and ctx->current_connection do not get cleaned up, which results in the next coro being deadlocked because we did not unlock pthread_mutex_unlock(&ctx->lock); (after obtaining the lock and checking if the streaming_client already exists.)

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