Dramatic acceleration of get-operations by mdbx_cache_get() API.

[erthink]

The essence of this "caching" is to check as quickly as possible whether the data has changed or not, including with early exit when searching though a b-tree.

At the same time, on the user's side, the minimal version information is stored in the MDBX_cache_entry_t structure, along with the offset to the "cached" data inside the memory-mapped database file.

Thus, instead of a full b-tree search, a minimum number of actions are performed, and the search stops when it reaches a database page that has not been modified after the last check. All together, it provides dramatic acceleration up to several tens of thousands of times.

In early 2026 the get-cached feature is ready and covered with tests an, including all cases of insertion/updating/remove of entries in a concurrently shared lock-free cache.