Module Tjr_lru_cache.Multithreaded_lru

Non-functorial multithreaded / concurrent-safe implementation of the LRU interface. This extends the in-memory explicit-state-passing code to the monad.

Compared to the in-mem version:

• we use the monad
• we allow multiple threads to call the interface methods
• for long-running operations (eg find, when the key is not in the i-m cache), we maintain a queue of blocked threads, make a single call to the lower layers, and when this returns we release the threads
• we have explicit messages to the lower layer
• the api includes "persist mode" (now or later) to indicate whether insert/delete operations should persist immediately or not

Given that the likely target is Lwt, this is probably overkill.