Improve in-memory cache for statistics:

• load and refresh the data from the persisted statistics. A simple example for NDV: https://github.com/mongodb/mongo/blob/master/src/mongo/db/query/compiler/ce/ndv/field_stats_loader.h
• Introduce memory limits
• Consider making it lock-free

Implementation: https://github.com/mongodb/mongo/blob/master/src/mongo/db/query/compiler/stats/stats_cache.h (also stats_cache_loader.h, stats_catalog.h in the same directory).