Each cache slot now contains a mutex. This mutex is locked in the OPEN

Each cache slot now contains a mutex. This mutex is locked in the OPEN
and SAVE commands and released after the command ends. This fixes the
cache mutex being locked for long periods of time and having
adjust_cache_time_thread() spawn threads that stacked until the cache
mutex was released.

The file_cache_t also contains a new member .refcount. This keeps
track of how many clients are using this slot so the cache modifying
commands don't overwrite the mutex.