Storages/Kafka: fixed deadlocks, moved to background pool #3215
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This fixes two deadlocks in Kafka engine found previously:
was never returned to the storage. Now it is claimed only
when actually starting reading.
but didn't close, and the endine then timeouted when waiting for
consumer destruction.
This also moves the stream thread to background worker pool.
The reason for that is that it will compete with other tasks for
time, so it will form a backpressure on insertion when the system
is busy.
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cc @bocharov @dqminh