Split SyncMetrics into network and sync parts

Currently, SyncMetrics are used by both the network layer and the sync layer. The idea would be to split that class into two pieces, so we won't need to pass the instance between these two layers.

Start by locating SyncMetrics and tracing its uses in the network and sync layers. Map which responsibilities belong to each layer and identify how the instance is passed between them. Done means the metrics are split into separate network and sync pieces without passing one shared instance between layers.

Written by the indexing model from the issue text.