High performance degradation on HierarchicalFilters update in Group::put

[ikopylov]

Performance degrade from 25000 RPS to 10000 RPS.
This can be connected with write lock acquisition or be a problem in add_to_parents.

https://github.com/qoollo/bob/blob/master/bob-backend/src/pearl/group.rs#L228

[ikopylov]

This is definetely due to the write lock acquisition. The operation under write lock does not change the collection structure, thus it can be a read lock. Actual update happened only in filters inside, so there should be an additional lock inside HierarchicalFilters that control access to actual filters