This optimization includes:
- all counters are stored in memory by default in order to minimize disc I/O operations
- counters that need to be persisted are flushed periodically to the persistent store
- switch to sync.Map which is more suitable for the case of disjoint sets of keys
- using lock-free data structures for the critical parts

This reverts commit 17f7837b.

Reverts this change due to a goroutine leak