This commit adds a new flag remoteWrite.inmemoryQueues, which starts a dedicated set of workers for processing only in-memory part of vmagent persistentqueue. It should help to mitigate an
issue when stale data at file-based queue prevents from ingestion recent data.
There is a downside - in case of remote storage is not reachable,
it's possible to get only a part of data ingested and other part queued
into file-based queue. But it should be acceptable, because there is no
strong guarantees for the data ingestion order.
Fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/8833
8480f6b43e