Optimize blobovnicza caching

[fyrchik]

There are multiple problems with the current scheme:

1. We have separate opened/active cache which leads to a complex implementation (mutexes should be taken in a specific order and we MUST take both if blobovnicza has a chance of being evicted).
2. Because of this separation opened_cache_size can actually be violated: actual amount of opened files is len(active) + opened_cache_size.
3. We can remove items from blobovnicza thus the property opened cache contains filled blobovniczas is no longer true.

I propose to refactor it like this:

1. Have only 1 cache with all blobovniczas.
2. Active map should only contain an index: when we need to put an item we just open blobovnicza if it is not already opened.
3. If width and depth parameters are very big compared to opened_cache_size, print a warning in logs.
4. *After active index exceeds width we should start again from 0 and, possibly, perform VACUUM.

[roman-khimov]

Obsoleted by #2453.