Time-sliced map store

[Snawoot]

This set of changes tries to improve MapLimitStore in order to get rid of full hash table scans in cleanup goroutine.

What's changed:

• Storage size counter is now maintained through bookkeeping to avoid iterations over time slices.
• Values are stored in tiered maps, each time slice is a separate map. It allows to expire all old values through simple deletion of old maps.
• Expiration logic was adjusted to guarantee values within expiration time will be retained. So we need to delete time slices with window which is twice older as expiration time to be sure.
• No last update time is associated with each value anymore.

[Snawoot]

Technicaly with that logic we could even get rid of background goroutine, doing cleanups in-band of Inc or Get calls. But I decided to leave it as it is for now to retain backwards compatibility.