With non-blocking pool restricted by 10 in capacity, the probability of
dropping events is unexpectedly big. Notifications are an essential part of the FrostFS,
we should not drop anything, especially new epochs.
```
Mar 31 07:07:03 vedi neofs-ir[19164]: 2023-03-31T07:07:03.901Z debug subscriber/subscriber.go:154 new notification event from sidechain {"name": "NewEpoch"}
Mar 31 07:07:03 vedi neofs-ir[19164]: 2023-03-31T07:07:03.901Z warn event/listener.go:248 listener worker pool drained {"chain": "morph", "capacity": 10}
```
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
